Environmental triggers and avoidance in the management of asthma
Received 1 September 2016
Accepted for publication 4 November 2016
Published 7 March 2017 Volume 2017:10 Pages 47—56
Checked for plagiarism Yes
Review by Single anonymous peer review
Peer reviewer comments 3
Editor who approved publication: Dr Luis Garcia-Marcos
Clarisse Gautier,1 Denis Charpin1,2
1Department of Pulmonology and Allergy, North Hospital, 2Faculty of Medicine, Aix-Marseille University, Marseille, France
Abstract: Identifying asthma triggers forms the basis of environmental secondary prevention. These triggers may be allergenic or nonallergenic. Allergenic triggers include indoor allergens, such as house dust mites (HDMs), molds, pets, cockroaches, and rodents, and outdoor allergens, such as pollens and molds. Clinical observations provide support for the role of HDM exposure as a trigger, although avoidance studies provide conflicting results. Molds and their metabolic products are now considered to be triggers of asthma attacks. Pets, dogs, and especially cats can undoubtedly trigger asthmatic symptoms in sensitized subjects. Avoidance is difficult and rarely adhered to by families. Cockroach allergens contribute to asthma morbidity, and avoidance strategies can lead to clinical benefit. Mouse allergens are mostly found in inner-city dwellings, but their implication in asthma morbidity is debated. In the outdoors, pollens can induce seasonal asthma in sensitized individuals. Avoidance relies on preventing pollens from getting into the house and on minimizing seasonal outdoor exposure. Outdoor molds may lead to severe asthma exacerbations. Nonallergenic triggers include viral infections, active and passive smoking, meteorological changes, occupational exposures, and other triggers that are less commonly involved. Viral infection is the main asthma trigger in children. Active smoking is associated with higher asthma morbidity, and smoking cessation interventions should be personalized. Passive smoking is also a risk factor for asthma exacerbation. The implementation of public smoking bans has led to a reduction in the hospitalization of asthmatic children. Air pollution levels have been linked with asthmatic symptoms, a decrease in lung function, and increased emergency room visits and hospitalizations. Since avoidance is not easy to achieve, clean air policies remain the most effective strategy. Indoor air is also affected by air pollutants, such as cigarette smoke and volatile organic compounds generated by building and cleaning materials. Occupational exposures include work-exacerbated asthma and work-related asthma.
Keywords: asthma, environment, triggers, avoidance, prevention
Over the past several years, a wide range of epidemiological, clinical, and experimental studies have led to major advances in the field of respiratory allergies. A wide array of risk factors play a multifaceted role, guiding the secondary prevention approach. In this review, we will not elaborate on potential protective factors associated with the “hygiene hypothesis”, rather we will focus on allergenic and nonallergenic triggers (Table 1).1 For each of these, after a short definition, we will review studies that suggest or prove their involvement, and when appropriate, we will then evaluate the outcomes of avoidance procedures.
Table 1 Self-reported asthma triggers as ranked by the proportion of participants who have experienced the trigger and by how frequently the trigger was reported
Note: Adapted from Price D, Dale P, Elder E, Chapman KR. Types, frequency and impact of asthma triggers on patients’ lives: a quantitative study in five European countries. J Asthma. 2014;51(2):127–135. © 2014 The Author(s). Published by Taylor & Francis.1
House dust mite (HDM) allergens
HDMs are ubiquitous pyroglyphidae that live in human dwellings. The mite’s gut contains potent digestive enzymes, notably proteases that persist in the feces, and these are thought to induce allergic sensitization and asthma. Their proliferation is favored by high indoor humidity and elevated temperatures. Mite allergens are classified into 33 groups, listed in the systematic World Health Organization (WHO) and the International Union of Immunologic Societies (IUIS).
Arguments in support of their involvement as triggers of asthma attacks include an association between HDM allergen exposure and asthma outcomes (and in some studies, a dose–response relationship), such as asthmatic symptoms and the requirement for medication in allergic asthmatic patients.2,3 The relationship with visits to the emergency department has provided conflicting results, as the association reported in earlier studies4 was not found in a more recent one.5
There have been numerous studies on avoidance procedures in regard to HDM and HDM allergens since their discovery in the late 1960s. These studies have led to meta-analyses. Four meta-analyses on HDM avoidance for asthma by Gotzsche and Johansen, which were published in 1998, 2001, 2004, and 2008, concluded that avoidance was of no benefit to asthmatic patients.6 Custovic and Wijk7 concluded that there is currently inadequate evidence to advocate allergen avoidance in adults, whereas these interventions may be of some benefit in children (Table 2). Platts-Mills has criticized these meta-analyses on the basis that this analytical approach is inadequate because allergen avoidance protocols are multifaceted and personalized and are hence difficult to standardize.8 Apart from these statistical shortcomings in the meta-analyses, there are also a number of biological issues that need to be considered. First, which biological parameter is the most representative of patient exposure? Tovey et al9 suggest that in-bed exposure accounts for only 9% of the total exposure. Second, dust collected from the homes of patients includes many other air contaminants besides HDMs and HDM allergens, such as bacteria and endotoxins, molds, glucans originating from mold membranes, mycotoxins, and microbial volatile organic compounds (Table 3).10 Thus, avoidance procedures directly aimed at HDMs may only provide partial measures. Furthermore, being sensitized to HDMs does not per se mean that HDM allergens play a role in the clinical expression of the disease. Finally, in the context of unhealthy buildings, of which there are many, the building itself generally has one or more defects that are responsible for high indoor humidity that leads to HDMs, mold proliferation, and release of their metabolites.10 Programs that have proven to be effective use a multifaceted approach including community health workers,11 environmental counseling, therapeutic education, and home remediation.12,13
Table 2 Practical measures for reducing dust mite and pet allergen levels: effect on allergen levels and the clinical effect on symptoms of rhinitis and asthma
Notes: Evidence level graded according to the Scottish Intercollegiate Guidelines Network recommendations: Ia, evidence obtained from meta-analysis of randomized controlled trials; Ib, evidence obtained from at least one randomized controlled trials; IIb, evidence obtained from at least one other type of well-designed quasiexperimental study; III, evidence obtained from well-designed nonexperimental descriptive studies, such as comparative studies, correlation studies, and case studies; IV, evidence obtained from expert committee reports or opinions and/or clinical experience of respected authorities. Reproduced from Custovic A, Wijk PG. The effectiveness of measures to change the indoor environment in the treatment of allergic rhinitis and asthma: ARIA update (in collaboration with GA(2)LEN). Allergy. 2005;60(9):1112–1115. Copyright © Blackwell Munksgaard 2005.7
Abbreviation: HEPA, High efficiency particulate arrestance.
Table 3 Fungal components and products relevant for indoor air sciences
Note: Reproduced from Nevalainen A, Täubel M, Hyvärinen A. Indoor fungi: companions and contaminants. Indoor Air. 2015;25(2):125–156. © 2014 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.10
Abbreviations: EPS, extracellular polysaccharides; IgE, immunoglobulin E; MVOC, microbial volatile organic compounds.
Microscopic fungi, or molds, represent one of the main groups of microorganisms present in all buildings. Although there are thousands of mold species, only ~80 indoor molds are thought to be responsible for adverse health effects in humans.
There have also been a number of epidemiological surveys in regard to this topic. The results of these have been summarized in meta-analyses14–17 and in reports from national18,19 and international health authorities.20 The association between dampness or mold and the impact on health is presented in Table 4. From this, it would appear that these factors are relevant.
There are few published interventional studies regarding dampness and mold in homes. They have been reviewed by Cox-Ganser.21 In one study, there were significantly fewer emergency department visits and/or hospitalizations among children in the remediation group compared to those in the control group. In another randomized controlled study performed in the UK, asthma symptoms and asthma medication use declined in the intervention group. A third study involved moving participants into an “asthma friendly, Breathe-Easy Home”. Children randomized to move into these houses had more asthma-free days, and fewer of them required health care visits compared to those in the control group.
There are as many dogs as cats, although cats are more allergenic than dogs.25
Pet allergy can readily be documented based on the clinical history. Skin tests for pet hair and danders (shed skin cells) are not very specific because some allergens from dogs cross-react with the major cat allergen, Fel d 1. Allergenic particles are small, ranging from 2 to 10 µm in size, which allows them to remain airborne and to stick to surfaces and clothing.26,27
Avoidance of cat allergens is hard to achieve. Patients who do not have a cat at home will nonetheless encounter cat allergens in many locations because these allergens are ubiquitous.28,29 For patients who have a cat at home, the advice is generally to find a different home for the cat. Then, cat allergen levels can be reduced, although even when this advice is followed, while most families fail to adhere to this advice.30 Other authors see little point in such measures since even low levels of cat allergens will still cause sensitized patients to react.31 When the cat remains in the home, basic avoidance measures do not decrease cat-allergen levels.32,33 Several pet companies have marketed the so-called “hypoallergenic cats”, although there is no scientific evidence to support this claim.34
Dog allergens are found in dog hair, shed skin cells, saliva, and urine. There are no differences between breeds in terms of allergy induction. Dogs are less often responsible for allergic reaction than cats.25 Avoidance counseling does not yield better outcomes than with cat allergies.35
Cockroach allergens are mainly found in the kitchens of low-income housing. The combination of cockroach allergen exposure and allergic sensitization contributes to asthma morbidity.36 The environmental control of cockroaches has recently been summarized by a Joint Task Force of three major professional allergy organizations.37 Strategies that effectively lower cockroach allergen levels result in a clinical benefit.38 They include keeping the kitchen clean, promptly washing the dishes, putting all foods in sealed containers, frequent disposal of the garbage, avoiding leaving food to sit out, and setting roach traps.
Like cockroach allergens, mouse allergens originate from mouse urine, shed skin cells, and hair follicles, and they are mostly found in inner-city dwellings. High levels of allergens may be found in schools and homes, but their relationship with asthma morbidity is still controversial.39,40 Integrative pest management has been associated with a reduction in allergen levels in the kitchen and in bedrooms.40 Outcomes in terms of a reduction in the use of health care provisions in children are also controversial.41,42 Public buildings in France have not been found to contain high levels of mouse allergens.43
Allergenic pollens are those that are small enough to be transported by the wind. With most patients, allergy to pollen induces rhinoconjunctivitis. Nonspecific bronchial hyperreactivity increases with grass-pollen allergy44 as well as with Parietaria pollen allergy during pollination.45
In the Copenhagen Allergy Study, it was estimated that a quarter of the subjects who were allergic to pollen also exhibited allergic asthma from pollen, while approximately one-half of the subjects with allergic rhinitis to animals or HDMs also exhibited asthma from animals or HDMs.46
Pollen avoidance is only relevant during the time that the offending pollen is airborne, which underscores the importance of a precise allergologic diagnosis. During this time period, patients are advised to – if at all possible – be outdoors only early in the morning or during the late afternoon, because humidity is higher then and the pollen particles tend to stick to anthers. In addition to this, car and home windows should be kept shut. Patients may be advised to turn on their air conditioner, to wear a pollen mask and wraparound glasses, and to take a shower and to wash their hair after being outdoors for an extended period of time. These recommendations follow common sense, although they have not been validated by control experiments.
Alternaria and Cladosporium species are considered to be major outdoor allergens responsible for sensitization, development of rhinitis and asthma, and asthma exacerbation, including life-threatening exacerbations. In contrast, xerophilic species of Penicillium and Aspergillus are involved in allergic diseases as indoor allergens. In regard to indoor allergens, the diagnosis of outdoor mold allergies is difficult because allergenic extracts are not well standardized and most fungal extracts exhibit broad cross-reactivity with taxonomically unrelated fungi.47
Viral infections have been implicated in most (>80%) asthma exacerbations in children.48 Such infections are sometimes referred to as the “September epidemic” due to their seasonal pattern.49 Rhinoviruses are the most commonly encountered, although other viruses, such as respiratory syncytial virus, enterovirus, coronavirus, and human metapneumovirus, can also be involved. Viruses interact with allergens to induce asthma exacerbation and hospitalization.50
Aside from keeping away from infected subjects, vaccination against the flu, and pharmacologic treatment during the first few days following contamination, there are no preventive treatments.
According to the Centers for Disease Control and Prevention in the USA, 21% of Americans with asthma are active smokers.51,52 Asthmatics who smoke are likely to experience a higher rate of hospitalization than asthmatics who do not smoke.53 Another study, performed with pregnant women, demonstrated poor asthma control and increased asthma exacerbations in active smokers and ex-smokers.54 Furthermore, inhaled corticosteroids, which represent the most effective currently available treatment for treating chronic asthma, have a reduced efficacy in smokers.55
McLeish et al56 evaluated smoking cessation in smokers with and without asthma. They concluded that these two groups did not differ in terms of the abstinence rate and smoking relapse, although they did show that smokers with asthma exhibited a slower rate of decline in nicotine withdrawal symptoms and craving over time. Bittner et al57 concluded that providing smoking cessation interventions during hospitalization for asthma exacerbations was an underutilized opportunity. Perret et al58 suggested several ways to improve smoking cessation strategies in asthmatic smokers, such as individualized interventions, insisting on “lung age”, providing long-term follow-up, and, in the case of adolescents, involving older peers to deliver asthma education.
The relationship between passive smoking and childhood asthma exacerbation and hospitalization is well recognized. In the USA, it is estimated that, on an annual basis, >200,000 asthma exacerbations are associated with passive smoking.59
The implementation of a public smoking ban in Scotland has reduced the rate of asthma exacerbations.60 A similar effect was observed in Minnesota (USA) as well as at the national level.61,62 This latter survey demonstrated a lower level of second-hand smoke exposure, a decrease in the odds of current asthma symptoms, and a lower risk of visits to the doctor due to severe asthma symptoms. A meta-analysis of 11 eligible studies published between 2008 and 2013 determined that hospital attendance for asthma patients decreased by 10% following the ban.63
Changes in gaseous and particulate outdoor air pollutants are associated with daily asthmatic symptoms, a decrease in lung function,64,65 emergency room visits, and hospitalizations for asthma attacks.66 The lag time amounts to 24 hours following the acute episode. In children with moderate persistent asthma, the occurrence of symptoms and the decrease in lung function are more pronounced in those patients who do not follow a maintenance therapy.67 Avoidance of outdoor air pollution is hard to achieve because there is no realistic way of avoiding exposure. National authorities do not recommend that asthmatic subjects remain indoors during periods of elevated air pollution because indoor air pollution can in fact be more harmful. When air quality index reaches a certain level, asthmatic patients, and eventually all individuals, should reduce the time spent outdoors and, in particular, avoid exertion in the polluted air.68 Other recommendations, such as wearing a dust mask and taking supplemental antioxidants, are unlikely to be effective.69 Thus, as stated in this review, sustained clean air policies remain the most effective way to reduce health effects that are related to air pollution.
The impact of volatile organic compounds that are encountered as indoor air pollutants on asthma control is still a matter of debate.70 The main indoor air pollutant is cigarette smoke. In Pittsburg, an industrial community with high outdoor air pollution, indoor activities, especially cigarette smoke, are responsible for a large part of the indoor exposure.71 Compared to cigarette smoke, e-cigarette vapor produces very small amounts of all by-products.72 Indoor chemical air pollution can be lowered by avoiding cooking-related pollutants, avoiding the use of sprays and other cleaning materials that generate volatile organic compounds (VOCs), choosing furniture and building material with low VOC emissions, and regular air exchange by natural or mechanical ventilation. These recommendations have been made available to the general public.73
An increase in daily temperatures correlated with an increase in emergency room department visits for asthma, especially in patients older than 65 years.74 However, cold temperatures also trigger asthma attacks. In general, the effect of cold weather appears to last for several weeks, whereas the effect of hot weather was more short term.75
High altitude has traditionally been associated with a reduction in mite allergen exposure, a lower rate of mite sensitization,76 and an improvement in the condition of asthmatics.77 However, more recent studies have suggested that other factors could also be involved in this improvement.78,79 Overall, airborne pollen grain and fungal spore concentrations are low, thus providing favorable conditions for those who suffer from pollen allergies.80
Work-exacerbated asthma can occur when current asthma symptoms are made worse due to work; it does not refer to work as the cause of asthma.81 This phenomenon has been reported to occur in 21.5% of asthmatics who work. Dust is the most commonly implicated agent of work-exacerbated asthma in the health care and education sectors, while for service jobs, smoke is the most common agent. Work-related asthma can be triggered by specific occupational agents. According to the French network of occupational physicians, there was a significant decrease in work-related asthma over the period 2001–2009, especially in the cases of aldehyde, latex, and isocyanates exposure, but, conversely, there was a significant increase for quaternary ammonium compounds.82
Avoidance is mainly based on occupational hygiene and the use of personal protective equipment. It also relies on an adequate knowledge of the potential triggers of work-related asthma by asthmatic patients. A Canadian web-based educational tool has been developed and successfully tested with a group of asthmatic patients.83
In a series originating from the Swiss Drug Monitoring Center, bronchospasm was reported in 2% of adverse drug reactions.84 Analgesics and nonsteroidal anti-inflammatory drugs were involved in 24% of these cases, anti-infectious in 18%, cardiovascular drugs in 11%, drug formulation agents in 9%, vaccines and immunoglobulins in 5.5%, plasma volume expanders in 5.5%, and other drugs in 27%. However, bronchospastic reactions should not be equated with asthmatic bronchospastic reactions because the pathogenic mechanisms underlying these reactions remain unclear.
Stress and emotional disturbances
Stress may enhance airway inflammation by modulating immune cell function through neural and hormonal pathways. Psychological triggers have been consistently associated with exacerbations and emergency treatments.85 In Puerto Rican children, caregiver depression, perceived stress, and child’s negative life events were associated with high trigger scores.86 Another survey concluded that anxiety may exacerbate the effects of disrupted sleep on childhood asthma control.87 A Cochrane Database Systematic Review only found two intervention studies, and it concluded that family therapy may be a useful adjunct to medication for asthmatic children.88
Although food allergy is often associated with asthma, asthma is not, in and of itself, a strong predictor for fatal anaphylaxis.89
Avoidance is based on not eating offending foods and, with selected patients and a suitably skilled medical team, specific hyposensitization.
Sulfites, used as preservatives and antioxidants, are found in many foods, such as pickled foods, dried fruits, and drinks such as beer, wine, and soft drinks. They can induce allergy-like symptoms such as wheezing, tightness, and cough in people with underlying asthma and allergic rhinitis.90
Recommendations from some of the national societies and national authorities
Official recommendations regarding asthma trigger identification and management vary widely between countries. Here, we present various recommendations, sometimes contradictory, originating from national societies or national authorities. For example, the US guidelines for management of asthma exacerbations encourage “trigger identification and avoidance instructions”, without providing more detailed instructions.91 The British Thoracic Society also recommends identification of triggers and emphasizes the role of passive smoking.92 The French Pulmonology Society provides a list of the main triggers.93 The American Academy of Allergy and Clinical Immunology recommends to avoid any trigger but does not mention how to do that.94
In regard to allergen avoidance, there are large discrepancies in the recommendations from National Societies. The American College of Chest Physicians recommends decreasing exposure to passive smoking, humidity and molds, pests, and noxious gas.95 The British Thoracic Society does not recommend the use of physical and chemical methods of reducing HDM levels in the home. It concludes that “studies of individual aeroallergen avoidance strategies show that single interventions have limited or no benefit” and suggest to implement “a multifaceted strategy more likely to be effective if it addresses all the indoor asthma triggers”.93 The British Society on Allergy and Clinical Immunology states that HDM avoidance might be beneficial in highly motivated patients, and clinical benefits are more likely with multiple interventions.96 The French Pulmonology Society emphasizes avoiding active and passive smoking.93 It suggests that a home visit for environmental inspection could be useful for severe asthmatic patients who have failed to gain control of their condition despite optimum treatment. The Allergic Rhinitis and its Impact on Asthma (ARIA) group suggests “not to use single chemical or physical preventive methods or their combination” but rather “multifaceted environmental control to reduce exposure in patients with allergy to HDMs”.97 Finally, the Global Initiative for Asthma (GINA) states that “Although allergens may contribute to asthma symptoms in sensitized patients, allergen avoidance is not recommended as a general strategy for asthma”. These strategies are often complex and expensive, and there are no validated methods for identifying those who are likely to benefit.98 In summary, no scientific society recommends basic HDM avoidance procedures.
Many triggers, both allergenic and nonallergenic, and their interactions influence the natural history of asthma. The attending physician’s task is to list them for each individual patient and to provide avoidance counseling. Educational programs are best suited to allow patient needs to be discussed and to provide answers to any question. However, in many instances, avoiding triggers is not a guarantee of success, and a multifaceted program should be implemented.
The authors report no conflicts of interest in this work.
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