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Cigarette Smoking |
There is little doubt of the pervasiveness of the cigarette smoking habit in the United States today. Approximately one third of the adult population in the United States is a regular smoker (1). While this is substantially less than the approximately 40 percent of the adult British population who smoke, American smokers tend to consume more cigarettes (33 per day) than their British counterparts (2). Unfortunately, all the information physicians and other scientists have amassed concerning the health hazards of smoking and its expense to society in general has done rather little to significantly reduce the number of people who smoke. From 1963 to 1978 in the United States, the percentage of male smokers has dropped from 54 to 41, while the percentage of female smokers has dropped from 43 to 40 percent. Smokers continue to engage in an activity that is hardly very glamorous (drawing smoke into one's lungs), is becoming rapidly more expensive, and is being more and more socially unacceptable. Only 20% of people who try to stop smoking actually succeed on their first attempt, while only about half succeed after multiple attempts (62). Why in the face of all of these reasons not to smoke do so many million Americans continue the habit? It is this question that we shall attempt to answer with this chapter. Two major theories have been advanced to explain the seemingly irrational behavior of the smoker; why the smoker continues with their habit despite the good evidence that it is so bad for their health (and may even be harmful to their family's health). First, there may be a physiologic dependency upon substances found in the cigarette, and the second is that smoking behavior is deeply learned and habitual behavior that has to be unlearned for successful smoking cessation. Physiologic Dependency. One is that smoking is really very similar to drug dependency such as with cocaine or heroin, in that there is a physiological dependency which develops. In other words, when the smoker attempts to discontinue, even for a very short period of time (such as several hours), many uncomfortable sensations are experienced such as increased heart rate, tremor, and increased anxiety; these symptoms are quickly reversed when a cigarette is smoked and are remarkably similar to those symptoms experienced by patients withdrawing from more dangerous drugs. This school of thought would say that the major reason why the smoker continues to smoke in spite of all reasonable evidence to the contrary is that they have developed a physiological addiction to something in the cigarette, most likely nicotine. Learned Dependency. The other major school is that smoking is really primarily a habit which has become continuously reinforced through the lifetime of the smoker. When the smoker experiences stress or conversely needs a lift, the cigarette serves him well. There is this continuous reinforcement several times during the day, every day of the week, every month of the year for years. Smoking becomes so much of the person's life style as putting on clothes in the morning or brushing the teeth at night. But uniquely, smoking is frequently used to reduce anxiety as studies have shown that people tend to smoke more during stressful conditions such as at work, rather than just after getting up in the morning. Smoking then becomes an effective mechanism whereby the person deals with the everyday stresses of life; without the cigarette, these stresses become more meaningful, difficult to deal with and perhaps even more real. The symptoms experienced by the smoker attempting to quit are not due to withdrawal from some drug such as nicotine, but rather are due to the person's feeling of increased emotional stress and inability to effectively handle it. And they would also propose that the cigarette is not merely a crutch for these patients either, but rather that there may be a psychological reason for the paring between a cigarette and relief of stressful situations. Indeed, recent experimenters have shown that cigarette smoking may affect the person's electroencephalogram or brain waves in a manner that may be indicative of an increased ability to handle stressful situations, and have a decreased awareness of fatigue or tiredness. Naturally, these two theories are not mutually exclusive. In other words, it might be that for some people, smoking is merely a habit, one that they have not given much attention to and might rather easily be able to stop if only they could put their minds to it. Some other people may actually be "hooked" on nicotine and depend upon its stimulatory effects to help get them through the day. And other people might be more dependent upon the physiological effects that smoking may have on the central nervous system which allow them to handle stressful situations more efficiently, and to experience fatigue and tiredness less readily than might otherwise be possible. We will next examine each of these alternative methods and examine whether there is reasonable evidence for them. How do people start smoking? What is the reason why young people first light up that first cigarette and pull raw often unfiltered smoke into their lungs? Most cigarette smoking recalling their first experiences with a cigarette recalls it as an unpleasant experience. About 7 percent of smokers aged 11 to 13 years reported that they smoked out of curiosity, while 62 percent agreed that they first began smoking to "show off or look big" (3). So, generally first time smokers do not smoke because of the pleasure of smoking, although that may eventually become the major reason why they continue smoking. Rather, the first cigarettes seem to be smoked out of peer pressure and curiosity rather than a hoped for initiation into adulthood. The influence of family. Considerable recent interest has centered around whether smokers may begin their habit more frequently if there is smoking in the immediate family. Does smoking by an important family member destigmatize smoking and make it more acceptable for the young person thinking about taking his first cigarette? Such does indeed seem to be the case. In a study by Bewley and Colleagues (4), the children from several counties in Great Britain were followed as to their smoking habits related to smoking in their immediate family. What was found was that 11 to 16 year old boys were more likely to become smokers if their fathers smoked, and girls were more likely to become active smokers if their mothers smoked. Whether or not their brothers or sisters also smoked seem to also have some influence upon their ultimate decision (5,6). Additionally, the example of a child's teacher may also be of some importance, especially to boys (7). The increased likelihood of smoking by a child if some significant older member of his world also smokes seems to be surprisingly great. Murray and Cracknell (6) also reported that frequently the cigarette smoking habit is not passively encouraged by these significant other members of the immediate family, but may even be rather actively encouraged. They demonstrated that in their children studied, 23 percent of the 11 to 14 year old smokers were sometimes given a cigarette by their parents and 30 to 40 percent by their brother or sister. However, this cannot be the entire story since there is such a
large number of children who begin smoking who world is not inhabited by
a smoking authority figure. In a study by Mausner and Platt (8),
32 percent of the teenage children of mothers who smoked were cigarette
smokers, while upwards of 23 percent of the children of non-smoking mothers
were smokers. Thus, there must be other significant reasons other than just imitation of the habits of an authority figure in their life.
Poor self image. Not surprisingly for the often troubled adolescent and young adult, self image can serve as a strong predictor for those who will assume the smoking habit. Those school-children who consider themselves "tough" are more likely to smoke when compared with those children who do not (9). In the American Cancer Society study (8), adolescents thought that 72 percent of adults smoked and that it would be an adult thing to do, whereas in reality onoly about one-third of adults smoke. Smoking and being adult are clearly paired in many children's minds and it is likely that smoking frequently is seen as a "rite of initiation" into adulthood to which every adolescent seems all too willing to enter. In addition to the above-mentioned reasons given by adolescents for their initiation into the smoking habit, there may be some factors over which they, and nobody else, has any control - their genetic makeup. While it is somewhat out-of-fashion to say that one's behavior can be at least partially determined by one's genes, such may indeed be the case concerning cigarette smoking. In adults, smokers differ from those who do not smoke in many different ways. For example, the personality of the smoker tends to be more of an extrovert (10,11). Additionally, they may be more "emotionally labile", anxious, or "neurotic" than non-smokers (11,12,13). Additionally, there is some evidence that these personality traits precede the initiation of smoking. cherry and Kiernan (15) also demonstrated that higher levels of extraversion and neuroticism at age 16 influenced the likelihood of the person becoming a smoker before the age of 25. While such personality factors such as extraversion and neuroticism
are thought to be determined at least in part by heredity, it is not too
surprising that genetic factors also seem related to smoking. Fisher
(15) and Friberg et al. (16) studied identical (monozygotic) and non-identical (dizygotic) twins with relation to their smoking behavior. These
studies were based upon the assumption that characteristics which are at
least in part related to heredity will have a higher incidence in monozygotic
twins than in dizygotic twins. In other words, the genes that code
for a certain behavior would be present in both monozygotic twins whereas
there would only be a 50-50 chance of a single gene being present in dizygotic
twins. Both the Fisher and the Friberg study found that genetic identity
increased the similarity of smoking behavior. This conclusion has
been reinforced by other more recent studies. Eysenck and Eaves (10)
performed a similar study in 1980 and demonstrated that 74 percent of identical
twins but only 50 percent of non-identical twins had the same smoking status.
Additionally, they demonstrated that when identical twins were reared apart
(thereby eliminating the influence of parental behavior), the conclusions
remained the same. Thus, the proponents of the theory that
at least part of the decision to begin smoking is determined by the genes
and not by one's environment, have a power study to support their conclusions. There has been much recent attention in the scientific literature as to whether smoking is a learned behavior. Certainly, it is clear that in the early stages of the smoking habit, the individual is merely imitating someone in authority or an activity that is hope will appear "grown-up". Nonetheless, there is good evidence that with the passage of years and with the waning of a desire to appear "grown-up", there are other reasons why the individual continues to smoke. Certainly, part of the reason might be a physiological addiction to the products in cigarette smoke - especially nicotine. However, there appears to be equally good evidence that a significant reason why many people continue to smoke is that smoking has become for them something so thoroughly engrained in their behavior that it becomes as natural as putting on shoes or brushing the teeth before going to bed. To understand the reasons why smoking can be considered a learned behavior, it is necessary to review some of the important concepts behind learning theory. A fundamental principle of learning theory is that behavior (in this case smoking) is controlled by the consequences of that behavior. For instance, in a negative sense, a child quickly learns not to put his hand on a warm stove since he will be burned, or to go near a vicious dog since he might be bitten. In the positive sense, that same child might learn that chocolate tastes good and so develop a craving for the stuff, or that going to bed late at night is good since he can be with his older siblings more. Thus, in both a positive and negative sense, a learned behavior (whether avoidance or seeking behavior) is learned by the consequences of that behavior. If the results of that behavior are bad, then the behavior will less likely be continued, and if the results of a behavior are good, then chances are greater for a repeat performance. Many scientists would argue that cigarette smoking is no different. In a purely physiologic sense, there is evidence that nicotine can stimulate pleasurable reward pathways in the brain constituting a possible positive reinforcement for the smoking behavior (39). By this analogy, regular smokers are like rats who continue to push a bar to receive electrical stimulation of certain pleasure centers in the brain. Additionally, as pointed out by Russell (40), each time the person smokes a cigarette, a pleasure imput is received. Since the average person smokes 20 to 40 cigarettes per day, the constant, repetitive stimulation of the individual's pleasure center over many years, it is easy to see how smoking might rapidly become almost addictive. In addition to the number of pleasurable associations an individual has with a behavior to be learned, another major factor determining whether that behavior will be learned with the closeness of these pleasurable sensations with the behavior. In other words, if a child is immediately reprimanded for throwing a baseball through a neighbor's window, the behavior will have less of a chance of repetition than if the parent waits for a few weeks. Thus it is with smoking. There is an almost immediate pleasurable sensation with each cigarette closely pairing the pleasure with the cigarette. Thus, there is no doubt in the person's mind that that pleasurable feeling was caused by the cigarette and might bear repeating. Other sources of positive non-physiologic reinforcement are possibly present. The adolescent seeking attention and acceptance into a social clique might become more welcomed with a cigarette hanging out of the mouth. The young child might feel more grown up and mature while puffing on the same cigarette that his older brother uses. It is hard not to smoke when everyone else in the world seems to be puffing away. Certainly, there are many other possibilities for positive reinforcement while smoking a cigarette. Negative reinforcement may also occur. We have already addressed
the possibility that smoking may prove calming to those who are easily
emotionally aroused, and conversely may prove stimulating to those whose
world is too passive. The avoidance of an unpleasant feeling or circumstances
constitutes negative reinforcement, and may be as powerful as positive
reinforcement to the cigarette smoker. Certainly, those who have
attempted to stop smoking might be able to attest to the negative consequences
of nicotine withdrawal. When the negative withdrawal consequences
of smoking cessation are eliminated by a cigarette, a powerful message
is sent to the brain that smoking makes you feel more comfortable, and
immediately reinforces smoking behavior. For whatever reason, once the person begins to smoke, there are probably further influences to keep him smoking. Cigarette smoking is a dirty, expensive, and increasingly socially unacceptable behavior that must be constantly reinforced to be continued. The argument relates as to exactly what this continuous reinforcer is. Namely, is it the nicotine found in every cigarette, or the release of tension associated with smoking in general. The importance of nicotine. While there is no doubt much confirmatory information concerning the relationship between personality and genetic background upon a subject's beginning a smoking career, there is also some information which suggests that at least some consideration must be given to the importance of nicotine. For example, Armitage (17) showed that the level of nicotine that can be found in a smoker's bloodstream and urine shortly after smoking a cigarette should be expected to have subtle, but important pharmacological effects. Since the smoker obtains these critical blood levels of nicotine every time a cigarette is smoked, it is not unreasonable to assume that these constant bouts of exposure may have some important physiological and perhaps even psychological effects. Additionally, as pointed out by Stepney (18), tobacco is relatively unique among a great variety of vegetable substances in that it does not produce a particularly pleasant taste or smell if smoked. The fact that tobacco is nonetheless preferentially smoked over all of these alternative vegetable substances lends credence to the supposition that there might be some active pharmacological agent in its smoke. Also, as pointed out by Stepney, despite the great plethora of low tar and nicotine cigarettes that have been present for at least ten years, the great majority of smokers continue to sue relatively high yielding brands delivering rather substantial amounts of nicotine. For instance, 80 percent of the cigarettes old in Britain in 1977 had nicotine deliveries of greater than 1.2 mg (19). The method that smokers use when they smoke a cigarette is also
an interesting learned behavior. Lee (20) pointed out that 77 percent
of men smokers report inhaling the cigarette smoke moderately or deeply,
while in women the percentage is only slightly less. This method
of smoke inhalation suggests that the intake of smoke (and therefore nicotine)
is the purpose of cigarette smoking and not just a byproduct of smoking.
The smell of taste of tobacco, manipulation of the cigarette, putting something
into the mouth, or sucking on something can all be done without drawing
smoke into the lungs. Inhaling the smoke is not a natural behavior
and is done at the cost of causing some initial discomfort. It is,
however, an extremely fast and efficient way of delivering nicotine into
the blood and from there to the brain.
Maintenance factors not involving nicotine. Recently, there
have been some studies published which cast some doubt upon the hypothesis
that nicotine absorption is the primary reason why cigarette smokers smoke,
for instance, nicotine is known to be absorbed from mucous membranes (such
as the inside lining of the mouth or lungs) so that it is possible that
some nicotine may be absorbed from the mouth and does not require deep
inhalation into the lungs. Cigar and pipe smokers, unless they have
been previous cigarette smokers, do not draw the smoke into their lungs,
they rather tend to just puff on the tobacco and do not inhale deeply.
It had been proposed by those favoring nicotine addiction as the reason
for continuation of the smoking habit that these smokers simply absorbed
nicotine from the mouth mucous membranes and that their motivation for
smoking was thus the same as for cigarette smokers. However, this
turns out not to be possible. Nicotine is much better absorbed when
it is in the acidic form, as with cigarette smoke. It is possible
that cigarette smokers may indeed absorb some of their nicotine from the
mouth because of the acidic nature in cigarettes. However, in cigars and
pipes, the nicotine that is present is not in the acidic form, but rather
in the alkaline or basic form. In this form, nicotine absorption
is nil, meaning that nicotine absorption is not the reason for the perpetuation
of their habit (21,22). Therefore, it has been proposed that there
are probably non-pharmacological reasons why these people, and probably
cigarette smoking people, continue in their habit.
Electroencaphalogram. One method used to analyze this association between cigarette smoking, nicotine, and stress has been the examination of the EEG or brainwave activity of subjects before and during smoking. In man, analysis of the EEG generally indicates that smoking and nicotine generally have an activating effect (26) while smoking cessation produces changes in the EEG indicative of a reduced arousal (27). However, the situation is not quite this simple, but rather the effect of smoking on the EEG to a large extent seems to depend upon the circumstances during which a cigarette was smoked. Smoking has a depressing activity upon the EEG when subjects were stressed (as with white noise), but might have a stimulating effect when the subject was bored in a situation of stimulus deprival (28). Personality. Some observers have attempted to define cigarette smoking habits in terms of whether the subject is an extrovert or an introvert. Ashton (29) demonstrated that the effects of smoking could be duplicated by intravenous injection of nicotine into the person's body, and a dose-response relationship could then be established such that small doses of nicotine had a stimulant effect on the EEG while larger doses had a depressive one. Thus, it might be hypothesized that those smokers who are extraverted might smoke to have a low stimulating dose of nicotine while introverted smokers might smoke more quickly to maintain a depressive effect on their EEG. There has been some experimental evidence supporting this relationship, although much more work is needed in this field (29). Improved mental ability. Since it might be possible that smokers use their smoking habit and nicotine exposure to maintain their personality status and alter their own EEG activity it might also be possible that they use cigarette smoking to influence their performance on the job. Certainly, this seems to be listed as one of the major reasons why smokers smoke on the job, as they feel it helps them to remain calm and to direct their energies to their work. The subject of smoking influence of performance in man has been extensively studied. Although smoking does not appear to have any significant influence upon tasks requiring only short term attention, there is some evidence that smoking may maintain performance in tasks requiring sustained vigilance and attention. This performance is enhanced not only when comparing smokers who are smoking during the test to smokers who are not smoking (30), but also between smokers and non-smokers (31, 32). This increased performance status has been demonstrated in terms of both reaction time and the accuracy of signal detection (28). Additionally, smoking has been demonstrated to counteract slowed reaction time and impaired mental arithmetic performance induced by alcohol intoxication (33). Some of the more interesting experiments concerning the relationship between cigarette smoking and ability to perform dull, boring tasks were done by Heimstra and associates in 1967 (71). They studied the mental ability of two groups of 20 smokers and one group of 20 nonsmokers in a study of smoking and simulated continuous driving for six hours. One of the smoker groups was allowed free access to cigarettes while the other smoker group was not. Driving errors were higher in the smokers who were not allowed cigarettes and the nonsmokers than in the smoking group. The results of the study indicated that smokers performed more efficiently when permitted to smoke than either nonsmokers or smokers not permitted access to cigarettes. Another study (72) evaluated two groups of smokers and a group of nonsmokers who monitored the second hand of a clock for brief pauses over an 80 minute time interval; one smoker group was permitted to smoke cigarettes at regular intervals, while the other smoker group was not. Analysis of data from this experiment indicated that both the nonsmoker group and the smoker group deprived of cigarettes experienced a significant decrement in their ability to detect brief pauses, whereas the smoker group with access to cigarettes did not. The same group performed another similar study to determine the effect of smoking on an auditory task. One group of twelve smokers performed the same auditory task on two occasions; one time with free access to cigarettes, and another time while smoking nicotine-free cigarettes after overnight abstinence from smoking. The auditory task consisted of listening to half second periods of white noise and detecting those periods of noise which also contained a faint tone. As with the visual vigilance task described previously, the smokers with free access to nicotine containing cigarettes performed much more efficiently than did the same subjects when smoking herbal, nicotine free cigarettes. Impairment of ability to perform complicated mental tasks due to nicotine withdrawal may last for long periods of time, long after the acute withdrawal effects (severe desire to smoke, decreased heart rate) had subsided (69). The more chronic impairment of higher mental function induced by nicotine withdrawal may be another reason why cigarette cessation can be so difficult. This greater vigilance to tasks requiring a long attention span in the smoker may be explained by the arousal effect produced by nicotine on the subject's EEG. However, in terms of learning, this high state of EEG arousal may not be good for learning new material High arousal appears to be actually disruptive of initial learning, although it may facilitate the consolidation of learning and therefore enhances long-term memory (28, 35). Thus, the learning of new information is greatly influenced by smoking. The initial learning of new information might be impaired, but once learned and placed into short-term memory, the consolidation and long-term recall of that information might be improved. Relief of Stress. Most smokers would agree that there is an association between smoking and stress. This is born out by the fact that smokers tend to smoke more heavily when under stress, and less heavily under non-stressful situation such as early in the day or late at night. However, it has only been recently that this assertion has been studied by scientists to discover whether or not it is valid. As we have already seen, the background information pertaining to this question lends it some credence. We have noted that smoking increases alertness so that perhaps workers would have less of a tendency towards tiredness, and that it improves alertness as judged by the improved reaction time. It would seem, therefore, that when the performance of a task is at the premium, then the reinforcing effects of smoking on behavior could be expected to reduce stress that might arise from real or imagined failure. Additionally, there is some evidence that smoking may reduce the disruptive effects that stress might have upon performance. In other words, while some stress may be a good thing, and may aid the worker in the timely performance of a task, too much stress may be actually disruptive and discourage accurate completion of that task. Animal experiments seem to bear this out. In one experiment, monkeys and rats were trained to press a lever for food over a period in which there was an occasional tone stimulus that was paired with an electrical shock. In other words, the animal came to know that when a tone was heard, there was soon going to be a painful shock. Eventually, the animal began to associate that tone with the shock so that it began to fear the tone as much as the shock. This association of an event (tone) with a painful stimulus (shock) is called a "conditioned response". The experimenter then went on to show that when the animal heard the tone, his lever-pressing behavior for food acquisition was disrupted. The experimenters (35) then found that the disruptive effect of the tone and electrical stimulus on food acquisition behavior could be markedly reduced by prior administration of nicotine. Unfortunately, similar experiments in man are few in number and
often poorly performed. One experimenter was able to show that subjects
were able to tolerate higher levels of electric shock when they were smoking
than otherwise, and that the effect was greater when a cigarette of high
nicotine delivery was used than with a cigarette with a low delivery (36).
Additionally, Heimstra (37) showed that drivers who were smoking during
a long and monotonous driving simulation task did not experience the same
degree of fatigue, and increased feelings of aggression that became apparent
in deprived smokers and non-smokers during the same task. Primary and Secondary Reinforcement. Primary reinforcement refers to that reinforcement which occurs primarily due to smoking a cigarette, such as possibly the alleviation of tension or acceptance into a social clique. Secondary reinforcement is a little bit different and refers to those stimuli which have become associated with the primary reinforcer - the cigarette. Some examples of secondary reinforcement might be merely the sight of a cigarette (hopefully in another person's hands), the smell of cigarette smoke, or (as exploited in every cigarette advertisement) the sight of somebody else enjoying a few puffs. These secondary reinforcement then call to mind the act of smoking and remind the smoker that it has been awhile since the last puff. The constant association of these secondary reinforcers with the primary reinforcers may make allow the secondary reinforcers to assume independent reinforcement properties independent of the primary reinforcers (41). In other words, through constant association with the "pleasurable" aspects of smoking, the mere smell of cigarette smoke may come to reinforce the act of smoking. Smelling the smoke from another person's cigarette may (perhaps unconsciously) bring to mind these "pleasurable" aspects of smoking and encourage the smoker to pull out the pack of cigarettes in his shirt pocket. This may explain why it is so much more difficult for a smoker to stop if his wife continues to smoke, or visa versa. The continuous secondary reinforcement from his wife's habit makes it very difficult for the husband to forget that he is a smoker. Pity the poor person who tries to stop smoking while all the co-workers smoke, especially during stressful situations! Positive and Negative Reinforcement. One reason why smoking may be so difficult to discontinue is that there is constant positive reinforcement. This reinforcement often comes from the cigarette itself, possibly from the anxiety relieving effects of the cigarette upon the person's EEG, and maybe even from friends and co-workers. This positive reinforcement is constant, day in and day out, as it accompanies every cigarette smoked. A large component of the positive reinforcement associated with cigarettes may be due their nicotine content. Experiments have shown that nicotine is a positive reinforcer; exposure to the effects of nicotine will encourage the further exposure. In one study, subjects were tested during a three hour session in which they could deliver to themselves either an injection of nicotine or a placebo injection after ten presses on a lever. When they were given nicotine, the injection rate increase; however, when they were given a saline placebo, the injection rate decreased. When they were given both nicotine and placebo at the same time, they chose nicotine over the saline (66,67). Tolerance develops to the positive reinforcement of nicotine, as with most other drugs. After repeated administration of nicotine very ten minutes, significant tolerance to nicotine develops after the sixth or seventh injection (68). There is, however, precious little negative reinforcement associated with smoking. Certainly, there may be a vague feeling that the smoker might be doing something harmful to their health (or family's); but after all, they probably feel rather healthy. It usually takes many years before symptoms of heart or lung disease to develop, if they develop at all. It is this immediate gratification associated with lack of certain punishment in the future that makes the negative reinforcement of cigarette smoking so ineffectual, especially in groups who rely upon their intellectual abilities. A questionnaire by Warnburton and Wesnes surveyed a group of university students as to their motives for smoking; 86% of the smoking cessation clinic, 83% of the university students, and 59% of hospital workers agreed that "smoking helps me think and concentrate" (70). Freudian Concepts of the Cigarette. Briefly, when consideration is given to the psychological non-pharmacological reasons behind the smoking habit, some consideration must be given to psychoanalytical hypotheses for completeness. Freud, the father of psychoanalysis, considered smoking as an "auto-erotic" manifestation of primitive sexuality (23), even though he was an ardent cigar user and eventually died of cancer of the nasopharynx). However, attempts to correlate adult smoking habits with childhood experiences, or with an "oral personality" have been largely unrewarding (24,25,26). Nicotine ((5)-3-(methyl-2-pyrroloidinyl)-pyridine) is a very complex chemical with many varied effects in the body. Nicotine is very toxic, and has even been used as an insecticide for many years. The lethal oral dose in humans is about 40 grams, the equivalent of approximately two packs of cigarettes (42). Absorption of Nicotine from cigarette smoke. Nicotine is absorbed very efficiently into the body from inhaled cigarette smoke. Approximately 90 percent of the inhaled nicotine is absorbed in the lungs with relatively little absorption occurring in the mouth (43,44). There is, however, a wide variation in the blood level of nicotine found among smokers. For example, after smoking one cigarette, pregnant women had peak blood nicotine levels ranging from 14 to 41 nanograms/ml. The wide variation in blood levels of nicotine is probably due to differences in smoking practices such as the depth and time of each inhalation, the number of puffs per cigarette, the brand of cigarette smoked, and how completely each cigarette is smoked. For example, the nicotine content of the final puffs of a cigarette contain about three times more nicotine than the first puff (42). Approximately 100 to 200 nanograms of nicotine are absorbed with each inhalation of tobacco smoke (43). This becomes important when it is considered that only 1 ng can produce physiological effects in the body (45). Finally, there is little correlation between the yield of nicotine from a cigarette as determined by the Federal Trade Commission (FTC) and the amount of nicotine absorbed from a cigarette because there are so many variables in how people smoke. For example, most low nicotine yield cigarettes have ventilation holes placed near the end of a cigarette to dilute the cigarette smoke with room air. However, these holes often become occluded by the smoker's lips or fingers, thereby producing a far greater nicotine yield than the cigarette manufacturer claims (57,58). Finally, the person's smoking practices such as their rate of smoking, how long the smoke is held in the lungs, how deeply the smoke is inhaled, also determine the nicotine yield of a particular cigarette to that person (59,60,61). Dispersal of nicotine through the body. After absorption into the blood stream, nicotine is then dispersed throughout the body. The brain, adrenal glands, and the pituitary glands are the three areas in the body which achieve the highest nicotine levels. Brain. Of course, the area of the body which vitally interests most smokers is the absorption of nicotine into the brain. Approximately 25 percent of the inhaled dose of nicotine passes directly into the brain (46). Later, nicotine is actively concentrated there, and nicotine concentrations commonly reach levels as much as eight times greater than that found in blood (47,48). Nicotine has an especially high affinity for certain area of the brain called the gray matter (compared to the white matter) and becomes especially concentrated in the hippocampus, diencephalon, and medulla. Recent research has attempted to characterize specific nicotine binding receptors in the central nervous system (63,64,65). It is hoped that someday it might be possible to produce medicine which will compete with nicotine for these binding sites in the nervous system that could aid in the treatment of tobacco dependence. Adrenal Glands. The other 75 percent of the inhaled nicotine dose passes to other tissues such as the adrenal glands. These glands are vitally important to the maintenance of health, and secrete many substances such as hormones, steroids, and other compounds. One of these compounds is epinephrine or adrenalin. Epinephrine is a very powerful substance which causes the heart to pump much harder and may increase the blood pressure. Concentration of nicotine within the adrenals causes them to release more epinephrine than otherwise, thereby possibly accelerating heart disease in the smoker and producing many adverse consequences in the fetus of the maternal smoker. The body attempts to get rid of the nicotine almost as quickly as it is introduced into the body, just as it would any other drug. The blood half-life (time for half of the introduced drug to become half destroyed) is about 40 minutes (43). The rapid disappearance of the drug from blood occurs mainly from uptake and active concentration into the various tissues (such as the brain and adrenal glands as described above) but also by the liver. The liver is the body's main gland for the detoxification of drugs and other toxins, and the excretion of them into the bile where they are often eliminated into the feces. The main metabolites of nicotine are cotinine (which has been used extensively to monitor compliance with smoking-cessation programs as it can be detected for days (42)), nicotine-1-N-oxide, hydroxycotinine, and nornicotine. Generally, it is thought that these secondary metabolites of nicotine are inactive, and that the physiologic effects from nicotine are due to the parent compounds (42). Elimination of nicotine is essentially complete within 24 hours of the last cigarette although there is some dose dependence. For example, if ten cigarettes are smoked within a two hour period, elimination of the ingested nicotine may require 48 hours (49). Of vital concern to the mother who smokes is the degree of nicotine
contamination of her milk if she continues to smoke. Certainly, it
has been known for a long time that nicotine passes into the milk, and
that the concentration found found there is related to the number of recent
cigarettes smoked (50-52). Additionally, it has been known since
the 1930's that there may be a substantial transfer of nicotine across
the placenta to the developing fetus. Animal studie sin mice have shown
that there was some transplacental nicotine transfer, although the concentrations
of nicotine in fetal tissues was considerably lower than in maternal blood.
This indicates that the placenta may represent an imperfect barrier to
the passage of nicotine into the developing fetus. Unfortunately,
the ability of the placenta to prevent nicotine transfer to the fetus might
not be the same in all animals (53). Additionally, the immature fetal
liver is not nearly as efficient as the adult liver in the breakdown of
nicotine, although liver activity increases markedly by the end of gestation
(54). More recent investigations have attempted to determine the
degree of nicotine placental transfer in humans. Van Vunakis and
co-workers (55) have reported that both nicotine and its degradation product
cotinine can be detected in amniotic fluid during the second trimester
in smokers, while no nicotine was detectable in the amniotic fluid of non-smokers.
Cotinine however, has been detected in the amniotic fluid of women exposed
to cigarette smoke as a result of passive smoking. This study was
particularly interesting since it was not performed in an artificial environment
but rather represented a much more common scenario. Andresen and
co-workers (56) detected cotinine in the amniotic fluid of two non-smoking
women whose husbands stayed at home much of the day and smoked. While
the cotinine levels were lower than that found in smoking women, it is
not at all clear that the levels are entirely non-toxic, especially to
a developing fetus.
References Nicotine
1. Surgeon General. The Changing Cigarette. US. Department
of Health and Human Services. Washington, DC., 1981.
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