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Document "Asthma, allergy and environmental factors": English Abstract
Asthma and allergy are very common disorders, which have major effects on an individual’s quality of life. Increases in asthma and allergy were reported during the final decades of the 20th century, particularly among children. This was not restricted to the Netherlands, as other western countries were similarly affected. With reference to this, the State Secretary for the Environment, has requested advice concerning the role of various environmental factors in the development and increase of these disorders. In response to the State Secretary’s questions, the President of the Health Council of the Netherlands appointed a committee, which based its advice mostly on existing reviews of the relevant literature.
Definition of concepts
Asthma is a chronic disorder that is characterised by recurrent episodes of wheezing, breathlessness, chest tightness, and cough, particularly at night or in the early morning. These symptoms are usually associated with airway responsiveness to stimuli, variable airflow limitation, and chronic inflammation of the airways. It is not always easy to diagnose asthma, partly due to the episodic nature of this disorder. Asthma can be categorised into allergic and non-allergic forms. These forms differ in terms of the mechanisms which give rise to them, and to some extent they require different preventive measures.
The type of allergy addressed by this advisory report is a hypersensitivity reaction that is based on the production of specific antibodies or immuno-globulins (IgE) in response to certain allergens. The predisposition to produce these antibodies is also referred to as atopy. The presence of a detectable level of IgE antibodies is referred to as sensitisation. The development of allergy symptoms is dependent on an individual’s hereditary predisposition, on the intensity and duration of their exposure to allergens, and on the degree of sensitisation involved. Allergic reactions of this kind can manifest themselves in the lungs (allergic asthma), in the eyes and nose (conjunctivitis and allergic rhinitis), or in the skin (atopic eczema). The first two examples are manifesta-tions of an allergy to inhaled substances (aeroallergens), accordingly they are also referred to as respiratory tract allergies.
Asthma and allergy in the Netherlands
The State Secretary’s first request to the Health Council was for advice concerning the validity and significance of current data for the prevalence of asthma and other allergic disorders of the respiratory tract in the Netherlands. He specifically wanted details of any changes in these data during recent years, asking the Health Council to assess whether the collection of these data have been sufficiently systematic to allow a conclusion to be reached concerning the actual increase in prevalence over time.
Descriptions of the prevalence of asthma and allergies in the Netherlands can draw on two types of data, one derived from registrations by general practitioners and the other from population studies.
On the basis of five registrations by general practitioners, the number of individuals with asthma in the Netherlands in 2003 was estimated at almost 520,000. The prevalence was 30 per 1,000 men and 35 per 1,000 women. Four to seven percent of primary school children exhibit symptoms of asthma. This means that asthma is currently the most common chronic illness among children in the Netherlands. Asthma, allergic rhinitis, and atopic eczema often occur in the same individual. Follow-up studies have shown that well over half of these children continue to suffer from these symptoms into adulthood. A Dutch study showed that about one in five adults exhibits a hypersensitive response to exposure to stimuli administered via the respiratory tract. One in every three adults has detectable levels of specific antibodies to commonly occurring aeroallergens. These are derived from house dust mites, pets, pollen and moulds.
Increases in asthma and respiratory tract allergy were reported during the final decades of the 20th century, both in the Netherlands and elsewhere. In recent years, the percentage of existing cases of the disease (prevalence) has remained the same, and may even be in decline. The Committee partly attributes the observed changes over time to changes in diagnosis. However, as diagnostic changes in various countries are unlikely to have taken place simultaneously, it mainly attributes the changes in prevalence to genuine changes in the percentage of new cases of the disease (incidence). The Committee advocates the use of the same diagnostic criteria in consecutive future studies, to facilitate the comparison of results. This approach will allow well-founded statements to be made about trends over time.
Monitoring of asthma and allergy
Secondly, the State Secretary asked the Health Council to investigate the nature and size of groups at risk of developing asthma and respiratory tract allergy. He asked the Council whether current monitoring programmes into the geographical and temporal occurrence of these disorders are suitable for monitoring their prevalence in high-risk groups.
The children of asthmatic or allergic parents form a high-risk group for the development of asthma and allergies. Another high-risk group consists of children who were either born prematurely or who had a low birth weight.
The monitoring of asthma and allergy can be described as the periodical measurement, analysis and interpretation of specific indicators of asthma and allergy. Individually, the current monitoring programmes provide too little information about changes in the prevalence of asthma and allergy in the population of the Netherlands. With some modifications, the Local and National Monitor of Young People’s Health, which is run by the Dutch Association of Municipal Health Services (GGD Nederland), the National Institute of Public Health and the Environment (RIVM), TNO Quality of Life, and an organisation of care entrepreneurs (ActiZ) could definitely yield insights into changes over time in children. It could also provide opportunities for early diagnosis and treatment. The majority of children with asthma have parents who do not suffer from asthma or allergy. The Committee therefore takes the view that it would be more useful to monitor asthma and allergy in the juvenile population as a whole than to monitor specific high-risk groups separately.
The role of predisposition and environment
Thirdly, the State Secretary asked the Health Council for details of the current level of knowledge concerning the role of – and interactions between – various genetic and environmental factors in the development and increase in asthma and respiratory tract allergy. He also asked the Council to focus specifically on the gaps of knowledge in this field.
Predisposition. Asthma and allergy have long been known to have a hereditary component. The children of asthmatic or allergic parents are at greater risk than other children of developing allergy or asthma. No single gene is responsible for this, instead a series of genes is involved. Regarding the fact that the genetic makeup of the population changes only gradually, any changes in prevalence – beside changes in diagnosis – can be largely attributed to changes in environ-mental and lifestyle factors.
Environment. Since the 1990s, various studies have been initiated into the role of environmental factors in the development of asthma and allergy in children. The factors that have been investigated include infections, non-infectious micro-biological components, allergens, breast feeding, dietary habits, overweight, premature birth, and air pollution inside and outside the home. To date, that research has led to the following conclusions.
Microorganisms. The effect of microorganisms on the development of asthma or allergy varies from species to species. Some studies have suggested that certain gastrointestinal infections reduce the risk of sensitisation. The intestinal flora may have a role to play in this. Conversely, other gastrointestinal infections tend to increase the risk of sensitisation. Little is known concerning the effect of respiratory tract infections on the development of allergic disorders. However, there is strong evidence to suggest that children who developed a respiratory tract infection caused by Respiratory Syncytial Virus (RSV) at an early age are at greater risk of developing asthma later in life. There is no indication of a causal relationship between vaccinations or the use of antibiotics at an early age, and the development of asthma or allergy.
Microbiological components. The relationship between exposure to non-infectious microbiological components in house dust (particularly to bacterial endotoxins ) and the development of asthma and allergy is a complex one. While there are indications that endotoxins inhibit allergic sensitisation, there is also evidence pointing to a link between these compounds and the development and aggravation of asthma. It is unclear whether the endotoxins themselves play a purely causal role here, or whether they serve as an indirect measure of exposure to other environmental factors, such as certain components of moulds. Recent research has identified major interactions with specific genes, which suggests that the relationship between endotoxins on the one hand and asthma and allergy on the other may differ markedly between individuals with different hereditary predispositions.
Allergens. The development of an allergy is always associated with exposure to specific allergens. Follow-up studies on newborns, in which actual measure-ments were made of their exposure to allergens during the early stages of life, have shown that exposure to allergens from house dust mites and cats in particular increases the risk of developing sensitisation. While sensitisation and asthma are linked, the effect of early exposure to allergens on the development of asthma is not yet fully understood. For the moment, there is little evidence to suggest that allergens shed by pests are of any significance. There is also a lack of clarity concerning the extent to which sensitisation to mould allergens contri-butes to the development of respiratory disorders. However, it is likely that infants with an allergy to cow’s milk allergens or chicken protein allergens will be at greater risk of other allergenic disorders when they reach primary school age.
Breastfeeding. Well-designed follow-up studies show that children who have been exclusively breastfed for at least three to four months are less prone to asthma and allergy. This particularly applies to the children of asthmatic or allergic parents. Various studies into the effect of breastfeeding on the develop-ment of asthma and allergy over the long term have yielded ambiguous results.
Dietary habits. Dietary habits also play a part in the development of asthma and allergy. For instance, there is evidence that dietary components in fruit, vegetables and oily fish may have a protective effect. Conversely, vegetable fats and salt have been reported to have harmful effects.
Overweight. Some researchers take the view that overweight increases the risk of developing asthma. However, the relationship is complex in nature, and there is insufficient evidence of a causal association.
Premature birth. Premature birth involves an increased risk of asthma symptoms and pulmonary function disorders in adulthood, but not of allergy. The mechanisms which give rise to respiratory tract symptoms probably differ from those involved in allergic asthma.
Outdoor air pollution. Asthma symptoms are generally aggravated by exposure to potent stimuli in the respiratory tract. Outdoor air pollution (from traffic or summer smog, for example) can also lead to the aggravation of asthma symptoms and other respiratory complaints. As yet, little is known concerning the extent to which long-term exposure to air pollution is also involved in the development of asthma and allergy.
Indoor air pollution. The major source of indoor air pollution is tobacco smoke. The 2003 Health Council advisory report entitled The impact of passive smoking on public health concluded that children whose parents smoke have an increased risk of developing asthma later in life. This is especially true if their mothers smoked during and after pregnancy. In the intervening years, none of the material published in this field has given cause to amend this report’s conclu-sions. Various studies have also revealed a relationship between living in a damp house and the risk of developing asthma or allergy. It is still not completely clear whether increased levels of damp in the home is an indirect measure of exposure to mite or mould products, or whether other factors present in the indoor air might be responsible for the observed effects.
The Committee concludes that hereditary predisposition in particular, together with exposure to specific allergens, plays a role in the development of asthma and allergy. Some microorganisms (or components thereof) may have a protec-tive effect. Premature birth and exposure to air pollution (including tobacco smoke) are probably involved in the development and aggravation of non-allergic asthma. Nothing is known concerning the relative quantitative importance of the various risk factors in terms of the development of asthma and allergy, nor of changes in prevalence over time.
Measures for the living environment
The State Secretary’s final question concerned preventive environmental measures. He asked which would be most effective (with reference to specific high-risk groups and phases of life), both in preventing the development of asthma and other respiratory tract allergies and in improving the condition of patients with asthma and respiratory tract allergies by reducing their symptoms.
In addressing this question, the Committee has focused on intervention studies in which the effect of measures to reduce exposure to environmental factors is experimentally investigated.
Amelioration of asthma or allergy symptoms. First the effect of measures to alleviate existing symptoms were evaluated.
House dust mites. Studies carried out in the Netherlands and elsewhere have shown that mattress covers that are impermeable to house dust mite allergens are limited effective in reducing the exposure of asthma and allergy patients to house dust mite allergen. However, this seldom produced any improvement in the health status of adult asthma and rhinitis patients.
Pets. The most obvious way to reduce pet allergens is to dispose of the pet to which the patient is allergic. However, few intervention studies have explored the efficacy of this measure, possibly because studies of this kind cannot be blinded. Special air filters cannot be recommended, given the inconclusive nature of the results obtained by the few studies that have been carried out with such devices. Nor has any systematic research been carried out into the effect of regularly washing the pet in question in order to reduce the subject’s exposure to allergen, or of excluding the pet from the bedroom or living room.
Hypoallergenic food. Mothers who consume hypoallergenic food throughout the period in which they are giving breastfeeding, may, as a result, reduce the severity of any atopic eczema suffered by their child. Further research is needed in this area.
‘Healthy’ house. A Dutch survey has shown that mattress material in mechanically ventilated homes contains statistically less house dust mite allergen than in naturally ventilated homes. The results of some small-scale intervention studies in Denmark and Finland suggest that asthma symptoms can be alleviated if the individual in question moves to a ‘healthy’ house, with such amenities as a balanced ventilation system. It is impossible to use blinding in this type of study. For this reason, and because the numerous changes involved in moving house cannot be individually evaluated, the significance of these studies is too limited to allow any recommendations to be made.
Combinations of interventions. The Committee concludes that, to date, interventions in asthma and allergy patients aimed at a single specific environmental factor or a single specific environmental measure, have been found to have a marginally clinical effect. Nevertheless, on the basis of the results of previous studies into substantial allergen avoidance, it is convinced that wide-ranging environmental interventions are indeed effective. Asthma, in particular, is a disorder that is determined by many factors. The associated disease burden can probably only be limited using combinations of interventions involving a range of environmental factors. In everyday practice, however, this is difficult to achieve and to study. In particular, the results of the US Inner-City Asthma Study, involving multiple interventions targeted at individual patients, support the view that tailored interventions and targeted behavioural support can indeed be useful. The Committee advocates further research in this area.
Preventing the development of asthma or allergy. During the past few years, research has also been carried out into the efficacy of environmental measures in preventing the development of asthma or allergy.
House dust mites and pets. The restriction of exposure to allergens produced by house dust mites and pets before birth and during the first year of life has been claimed to reduce the risk of children with a predisposition for allergy develop-ing asthma and allergy. However, the results of studies carried out to date are far from unambiguous. One British study suggested that a stringent reduction of allergens can actually boost sensitisation to house dust mites.
Diet during pregnancy and infancy. At the age of eighteen months, children whose mothers consumed only limited quantities of food allergens during pregnancy are at no less risk than other children of developing atopic eczema. It remains to be seen whether excluding food allergens from the diets of infants themselves is of any use in preventing allergic disorders. The same applies to the benefits of using infant formulas containing non-infectious bacteria, such as lactobacilli. It has yet to be satisfactorily demonstrated whether such products, which are referred to as probiotics, actually inhibit the development of asthma and allergy.
Combinations of interventions. The Committee concludes that no interventions targeting a single environmental factor have proved to be sufficiently effective in preventing the development of asthma or allergy. Interventions involving restricted exposure to allergens, tobacco smoke, and specific dietary components may be more effective. This involves combinations of interventions targeting a range of environmental factors.

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