CAUSES AND TRIGGERS
Factors Affecting Asthma
A wide variety of factors are known to affect asthma, but no one specific cause, either biological or environmental, has been identified. Studies indicate the contribution of both genetic and non-genetic factors. When considering nongenetic factors affecting asthma, it is important to distinguish between the triggers of asthma attacks (which are widely recognised) and the causes of the underlying asthmatic trait (about which much less is known). Both groups of factors may contribute to the severity and persistence of asthma.
Genetics: One part of the picture
Asthma often runs in families, and identical twins are more likely to both be asthmatic than are non-identical twins. Nevertheless, only about half of the identical twins with an asthmatic co-twin are themselves asthmatic, indicating a contribution from both genetic and non-genetic factors.
Large studies of asthma in the general population have recently identified a small number of genetic variants that influence asthma risk, mainly in children. These variants are frequently found in populations of European origin, but their association with asthma is too weak to predict reliably which individuals will develop the disease.
Common triggers: The common cold and exercise
Asthma attacks are commonly triggered by upper respiratory tract infections, including common colds, and by exercise. Less frequently, they are related to acute emotional stress or to the consumption of certain foods, beverages or medicines.
Environmental factors that may provoke asthma attacks include inhaled allergens (commonly dust mites and animal fur; less commonly pollens, moulds and allergens encountered in the workplace) and inhaled irritants (cigarette smoke, fumes from cooking, heating or vehicle exhausts, cosmetics and aerosol sprays).
Environmental factors: Facts and theories
Secondhand smoke is a confirmed risk
Secondhand tobacco smoke is associated with asthma risk both in childhood and adulthood. Pre-natal exposure may also be important. This is considered to be a causal association, implying that the prevalence (and severity) of asthma would be reduced if exposure to secondhand smoke could be reduced. The role of other indoor air pollutants as causes of the asthmatic tendency is less clear and less consistent than for tobacco smoke.
Link to mould and damp is uncertain
Dampness and mould growth are more common in the homes of asthmatic children and adults. However, the causal nature of this link remains uncertain, because few asthmatic individuals are demonstrably allergic to fungal moulds, and dampness is associated with both allergic and nonallergic forms of asthma.
Animals in the home and on the farm
Exposure to furry pets is often less common among asthmatic children and adults, due to avoidance or removal of pets by allergic families. When this is taken into account, there is no consistent evidence that pets are either a risk factor or a protective factor.
In contrast, several large studies, mainly in temperate countries, have shown a lower prevalence of asthma among children living on farms. These children also have fewer allergies, but this does not totally explain the apparent protection against asthma. No specific cause has been identified for this protective effect of farm upbringing, but diversity of microbial exposure may be an underlying factor.
Antibiotics and paracetamol: cause or effect?
Asthmatic symptoms are more common among children who were treated with antibiotics in early childhood. However, the direction of cause and effect here is uncertain. Symptoms of wheezing commonly develop for the first time in infancy and may be treated with antibiotics before they are recognised as the early manifestations of asthma.
Similar considerations of “reverse causality” apply to the possible link between paracetamol (acetaminophen) exposure in infancy and asthma at school age – paracetamol may have been given for early symptoms of asthma, or for infections that may themselves increase the risk of asthma. Recent paracetamol use by adolescents and adults is also more common among those with asthmatic symptoms, but this may also be “reverse causality” in that people with asthma symptoms may avoid using aspirin, since it is a known trigger of wheezing attacks in a small proportion of asthmatics.
Preventive and remedial measures
Eat a balanced diet
Prolonged exclusive breastfeeding was once thought to protect against allergic diseases, including asthma, but this is no longer thought to be the case. Many components of diet during later childhood and adult life have been studied in relation to asthma. The balance of evidence suggests that “prudent diets”, those that are widely recommended to prevent cardiovascular diseases and cancer, are unlikely to increase the risk of asthma and may reduce it slightly.
Avoid exposure to causal agents
Occupational exposures provide some of the clearest examples of remediable causes of asthma. Occupational asthma may develop in persons with no previous history of chest disease and can sometimes persist after exposure to the causal agent is removed. High risk occupations include baking, woodworking, farming, exposure to laboratory animals, and use of certain chemicals, notably paints containing isocyanates. Perhaps the most widespread “occupational” exposure is to chemical cleaning agents, both in workplace and domestic settings.
Little is known about the factors affecting asthma after middle age, when there is substantial overlap between the reversible airflow obstruction, which is typical of asthma, and the irreversible airflow obstruction of chronic obstructive pulmonary disease (COPD). Active smoking is a major and remediable cause of COPD and probably contributes to some cases of adult-onset asthma. It should therefore be discouraged among both asthmatics and non-asthmatics alike.
Asthma and Air Pollution
One characteristic of asthma is that the airways are more reactive to a range of environmental stimuli, and it is reasonable to assume that asthmatics would be adversely affected by inhaling airborne pollutants. This has been the subject of a large amount of experimental, clinical and epidemiological research that suggests that, in the real world, air pollution plays a role in exacerbating symptoms in asthmatics. It is not clear however that air pollution causes the asthmatic disease itself.
Exacerbation, not cause
The question is complicated not only by the need to distinguish between exacerbations of existing asthma and the incidence of the disease asthma, but also by the complex nature of air pollution. In the outdoor environment, air pollution may arise from a variety of sources, the most important being from the combustion of fossil or biomass fuels. In the indoor environment, the most common sources are also from combustion with the main processes being cooking or tobacco smoking. The pollution mixture comprises particulate matter with a wide range of physical and chemical characteristics and a range of gases, including nitrogen dioxide, sulphur dioxide and ozone. The study of asthma and air pollution is constrained by the lack of good measures of asthma and by the lack of personal exposure data.
Outdoor air pollution: The impact of traffic
There have been substantial trends in asthma and in outdoor air pollution throughout the world, but overall these do not correlate with one another. For example, asthma has tended to increase in developed countries while, over the same time period, there have been very large overall reductions in outdoor air pollution concentrations.
While this negative correlation over time suggests that air pollution is unlikely to be responsible for asthma trends, it should be noted the increase is correlated with increases in traffic-related pollution. In cross-sectional studies, variations in asthma prevalence are not associated with variations in community-wide average concentrations of air pollutants. However, there is evidence from within-community studies that the prevalence of asthma symptoms is higher in children living near sources of traffic pollution. With rapid urbanisation across the world, and poor pollution control and vehicle maintenance in poorer countries, asthma related to traffic pollution may become more important in the near future.
In contrast to the rather weak and conflicting evidence concerning prevalence and incidence, there is more convincing evidence that air pollution can exacerbate existing asthma. This evidence is based on analyses of hospital admissions data and panels of asthmatic patients. However, the effect of air pollution on asthma exacerbations is relatively small in comparison with that of other exacerbating factors, especially respiratory virus infections.
Asthmatics are often sensitive to inhaled allergens, such as from house dust mites, and laboratory studies suggest that allergens interact with air pollutants to exacerbate symptoms. It is not clear whether this happens in real-world situations. Apart from asthma, it should be emphasised that there is quite strong evidence that air pollution causes respiratory symptoms and exacerbates upper and lower respiratory infections in children.
Indoor air pollution: The impact of tobacco and solid fuels
For children, one of the most important exposures to pollution indoors is secondhand smoke from parental tobacco smoking. This is somewhat easier to study than outdoor air pollution because better information on personal exposure can be obtained by asking about smoking in the home or measuring cotinine in biological samples, such as saliva.
The evidence from a large number of studies worldwide is that secondhand smoke is a cause of wheezing illness in early childhood, probably interacting with acute respiratory virus infections. This wheezing illness tends to be mild and transient, and the association with secondhand smoke becomes progressively less apparent in the school years. There is also evidence associating secondhand smoke with the onset of asthma in later childhood.
The other major source of indoor air pollution is from combustion of cooking fuels. About half of the world's population is exposed to smoke from cooking with solid fuels, often in poorly ventilated conditions. This exposure is associated with increased respiratory symptoms in children but there is little evidence to suggest that there is an effect on asthma incidence. In the developed world, gas cooking is the main source of cooking pollution, and this differs from exposure to solid fuel pollution in that particulate concentrations are much lower. The evidence of a large number of studies is not entirely consistent but suggests that gas cooking is associated with respiratory symptoms in children with asthma, and an increased severity of asthma associated with respiratory viral infections. Broadly, the evidence from studies of outdoor and indoor pollution points in the same direction; that air pollution plays a role in causing symptoms in children with asthma, but has an uncertain role in causing healthy children to become asthmatic.