Parental smoking (for children) and occupational exposures (for adults) are the clearest examples of remediable causes of asthma.
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 non-genetic 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 process or 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.
The role of allergy?
Asthma used to be thought of as an allergic disease, where allergen exposure causes sensitisation to allergens and continued exposure leads to the processes in the airway which lead to asthma symptoms. While allergy is a potential underlying factor for up to half of the people with asthma, the remainder have no allergic features. In low- and middle-income countries the proportion of people with non-allergic asthma is greater than in high-income countries. Furthermore, some occupational causes of asthma do not appear to involve allergy. These non-allergic mechanisms are currently not well understood.
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 tobacco smoke exposure, 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), and medicines (including aspirin).
Causes of the underlying asthma trait - environmental factors: Facts and theories
Environmental factors are much more likely than genetic factors to have caused the large increase in the numbers of people in the world with asthma, but we still do not know all the factors which may be important and how they interact with each other.
Secondhand smoke is a confirmed risk
Secondhand tobacco smoke has been confirmed as a risk for asthma both in childhood and adulthood (see references at the end of the report). 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, such as cooking on an indoor open fire, 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, inviting further research. Few people with asthma are demonstrably allergic to fungal moulds. Dampness in homes is associated with both allergic and non-allergic 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?
Asthma 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 asthma symptoms, but this may also be “reverse causality”; people with asthma symptoms may avoid using aspirin, since it is a known trigger of wheezing attacks in a small proportion of asthmatics, who use paracetamol instead.
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.
Preventive and remedial measures
Eat a balanced diet
Prolonged exclusive breastfeeding was once thought to protect against allergic diseases, including asthma, but extensive research has shown that this is not 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 diets that are widely recommended to prevent cardiovascular diseases and cancer may slightly reduce the risk of asthma. A link has been established between obesity and asthma, although the mechanisms are not clear.
Avoid exposure to causal agents
Occupational exposures provide some of the clearest examples of remediable causes of asthma. Special care is required in high-risk occupations (baking, woodworking, farming, exposure to laboratory animals, and use of certain chemicals, notably paints containing isocyanates) to minimise inhalation of potentially harmful substances, and care to reduce exposure to chemical cleaning agents in the home is also needed.
Don’t smoke or go near second hand smoke
Smokefree environments are important for people of all ages. 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. Smoking should therefore be discouraged among both asthmatics and non-asthmatics alike.
Environmental factors are much more likely than genetic factors to have caused the large increase in the numbers of people in the world with asthma. Tobacco smoking and secondhand tobacco smoke are avoidable by the individual. Occupational exposure is a risk diminishable by both workplace practices and government policies. These and other factors require further research.
Governments should strengthen policies to reduce tobacco consumption, encourage healthy eating, and reduce exposure to potentially harmful chemicals, smoke, and dust. Funders need to support further research to identify causes of asthma.