There have been a number of major reviews of the scientific evidence on the health effects of passive smoking. Most important are those of the International Agency for Research on Cancer (a branch of the World Health Organization (1985),(8) Australia's National Health and Medical Research Council (1986),(9) the US Surgeon General (1986),(6) the US National Research Council (1986),(10) the Independent Scientific Committee on Smoking and Health from the United Kingdom (1988),(11) and in 1992, the reports of the US Environmental Protection Agency(12) and the Royal College of Physicians of London.(13) Key findings from some of these reviews are reproduced below.
The NH & MRC reached the following conclusions about aspects of passive smoking:
1. Effects on child health
Council notes that smoking during pregnancy decreases birth weight and increases perinatal mortality. There is strong evidence to suggest that maternal smoking after birth is associated with increased diseases of the lower respiratory tract in the first year of life.
There is considerable information available which implicates parental smoking as a significant causal factor contributing to middle ear effusions and reduced lung function. Passive smoking may trigger asthma attacks in children who suffer from asthma and may also increase the risk of Sudden Infant Death Syndrome. Children of smoking mothers have a reduced attained height throughout childhood. However, further studies are required to clarify whether these effects, particularly impaired growth and reduced lung function, are due to exposure in utero or to passively inhaled smoke in infancy or to the continued effect of passive smoking through childhood.
2. Adult respiratory effects
Council notes that the epidemiological evidence shows that inhalation of passive smoke by healthy individuals and those with pre-existing respiratory disease commonly causes acute irritant effects in the upper and, to a lesser extent, the lower respiratory tracts. There are sufficient data to indicate that asthmatics may suffer significant acute effects following exposure to passive smoke. However, there is insufficient evidence to assess the effects of passive smoking upon susceptible individuals who have airways hyper-reactivity or atopy but who are not clinically asthmatic.
Council notes that, in several other important areas, there is insufficient scientific evidence yet available. In particular:
the effect on lung function of acute exposure to passive smoke in healthy individuals appears not to be substantial nor is the evidence consistent;
few data are available regarding the effects upon both the upper and lower respiratory tracts of chronic passive smoking in healthy individuals; and
there are no data on the effects of passive smoking during childhood on subsequent lung function in adulthood. Research in these several areas warrants encouragement and support.
3. Cancer
Council notes that there is mounting epidemiological evidence that passive smoking may increase the risk of occurrence of lung cancer. Despite limitations in the amount of data available, and despite research difficulties in making satisfactory estimations of individual exposure, a consistent pattern of moderately increased risk of lung cancer in passive smokers has emerged. In view of this pattern, of the known and substantial increase in risk of lung cancer in active smokers (and the lack of a threshold dose), and of the documented levels of bodily assimilation of passively-inhaled smoke, it is therefore prudent public health policy to infer an increased risk of lung cancer from passive smoking.
Council notes that the epidemiological evidence that passive smoking is associated with an increased risk of cancers at sites other than lung is less substantial and less consistent than for lung cancer.
Council also notes that the International Agency for Research on Cancer (a branch of the World Health Organization) has recently concluded that 'passive smoking gives rise to some risk of cancer'.
4. Cardiovascular Disease
Council notes that, while there is clear epidemiological evidence of markedly increased rates of myocardial infarction, re-infarction, and sudden (cardiac) death in active smokers, there is very limited evidence available about the cardiovascular effects of passive smoking. While two follow-up (cohort) studies of non-smokers have reported moderate increases in the risk of coronary heart disease mortality associated with passive smoking, there is an urgent need for further research into this potentially important public health question. This is particularly so in view of recent research indications of effects of low dosage of tobacco smoke and its constituents upon cardiac haemodynamics, particularly in persons with pre-existing coronary heart disease.
This report reached three major conclusions:
1. Involuntary smoking is a cause of disease, including lung cancer, in healthy non-smokers.
2. The children of parents who smoke compared with the children of non-smoking parents have an increased frequency of respiratory infections, increased respiratory symptoms, and slightly smaller rates of increase in lung function as the lung matures.
3. The simple separation of smokers and non-smokers within the same air space may reduce, but does not eliminate, the exposure of non-smokers to environmental tobacco smoke.
Major conclusions:
Based on the weight of the available scientific evidence, the US Environmental Protection Agency (EPA) has concluded that the widespread exposure to environmental tobacco smoke (ETS) in the United States presents a serious and substantial public health impact.
1. Effects in adults:
ETS is a human lung carcinogen, responsible for approximately 3,000 lung cancer deaths annually in US non-smokers.
2. Effects in children:
ETS exposure is causally associated with an increased risk of lower respiratory tract infections such as bronchitis and pneumonia. This report estimates that 150,000 to 300,000 cases annually in infants and young children up to 18 months of age are attributable to exposure to ETS.
ETS exposure is causally associated with increased prevalence of fluid in the middle ear, symptoms of upper respiratory tract irritation, and a small but significant reduction in lung function.
ETS exposure is causally associated with additional episodes and increased severity of symptoms in children with asthma. This report estimates that 200,000 to 1,000,000 asthmatic children have their condition worsened by exposure to ETS.
ETS exposure is a risk factor for new cases of asthma in children who have not previously displayed symptoms.
The EPA report classifies ETS as a Group A carcinogen under EPA's carcinogen assessment guidelines. This classification is reserved for those compounds or mixtures which have been shown to cause cancer in humans, based on studies in human populations.
Note: The EPA did not consider evidence concerning ETS and heart disease and other cancers.
Summary of facts prefacing Chapter 2: Passive smoking and the health of children.
1. Children of parents who smoke inhale nicotine in amounts equivalent to their actively smoking 60-150 cigarettes per year.
2. Over one-quarter of the risk of death due to the Sudden Infant Death Syndrome (cot death) is attributable to maternal smoking (equivalent to 365 deaths per year in England and Wales).
3. Infants of parents who smoke are twice as likely to suffer from serious respiratory infection.
4. Symptoms of asthma are twice as common in the children of smokers.
5. One-third of cases of 'glue ear', the commonest cause of deafness in children, is attributable to parental smoking.
6. Children of parents who smoke more than ten cigarettes per day are shorter than children of non-smokers.
7. Passive smoking is an important cause of school absenteeism, accounting for one in seven days lost.
8. Parental smoking is responsible for at least 17,000 admissions to hospital each year [in the United Kingdom] of children under the age of five.
9. Passive smoking during childhood predisposes children to developing chronic obstructive airway disease and cancer as adults.
10. Maternal smoking during pregnancy and infancy is one of the most important avoidable risk factors for infant death.
The Californian EPA report, released in 1997, was the subject of peer review, public comment and revision. It affirmed the findings of the US EPA on the link between environmental tobacco smoke and lung cancer and respiratory illness. It also found that there is a causal relationship between passive smoking and sudden infant death syndrome, heart disease morbidity and mortality, nasal sinus cancer, as well as serious chronic diseases such as childhood asthma.
It also found that there were effects for which evidence is suggestive of an association but further research is needed for confirmation. These include spontaneous abortion, cervical cancer, and exacerbation of asthma in adults.
This report was finalised in November 1997 and released without the recommendations which were included in the draft report. This report was the subject of legal action by the Tobacco Institute of Australia which delayed the release of the report.
The summary of findings
Asthma in children
It is estimated that children exposed to environmental tobacco smoke are about 40% more likely to suffer from asthma symptoms compared to children who are not exposed. It estimates that about 8% of childhood asthma is attributable to passive smoking. It is estimated that passive smoking contributes to the symptoms of asthma in 46,500 Australian children per year. The effect is most marked in children of mothers who smoke more than 10 cigarettes per day.
Lower respiratory tract illness
It is estimated that children exposed to environmental tobacco smoke during the first 18 months of life have a 60% increase in the risk of developing lower respiratory illnesses such as croup, bronchitis, bronchiolitis and pneumonia.
It is estimated that 13% of lower respiratory illness in Australian children under 18 months of age, or 16,300 cases per year, is attributable to passive smoking.
Lung cancer
It is estimated that people who never smoke and live with a smoker have a 30% increase in the risk of developing lung cancer compared with people who never smoke and live with a non-smoker. On this basis, it is estimated that exposure to a partner who smokes a home causes about 12 new cases of lung cancer and 11 deaths from lung cancer each year in people who never smoke. This estimate does not take in to account exposure outside the home, nor effects of passive smoking on ex-smokers or current smokers.
Major coronary events
It is estimated that the risk of heart attack or death from coronary heart disease is about 24% higher in people who never smoke but live with a smoker, compared with unexposed people who never smoke. Based on this estimate passive smoking could cause some 77 deaths and 132 hospital admissions for a major coronary event in Australia.
However, based on the available evidence, it could not be concluded with certainty that passive smoking is a cause of heart attack or death from coronary heart disease or any other type of cardiovascular disease.
Other illnesses
The review of the evidence also concluded that passive smoking contributes significantly to the risk of sudden infant death syndrome and may increase the risk of death from all causes.