Update on Indoor Allergens and Their Impact on Pediatric Asthma

Abstract

Objective:

In recent decades, many indoor allergens have been identified, including dust mite, cat, dog, mouse, cockroach and indoor molds, which have important health effects particularly in sensitized asthmatics. This review aims to update our understanding regarding the extent of these exposures in the indoor environment, review strategies for reducing their levels in the environment, and highlight innovative recent trials targeting these exposures and their impact on pediatric asthma morbidity.

Data Sources:

Recent practice parameter updates on indoor allergen exposures, seminal studies, and recent peer-reviewed journal articles are referenced.

Study Selections:

This review cites recent cohort studies of well-characterized pediatric asthmatic patients as well as innovative randomized controlled trials evaluating exposure to environmental allergens, interventions to limit these exposures, and their outcomes.

Results:

Links between indoor aeroallergen exposures and health outcomes have been well established. However, only some allergen reduction interventions have been successful in improving health outcomes.

Conclusion:

There are many complicating factors involved in allergic exposures and health outcomes. The interplay between patient genetic factors, indoor allergic triggers, airborne irritants/pollutants and microbial exposures complicate the study of indoor allergen exposures and their impact on asthma morbidity.

INTRODUCTION

More than 80% of children with asthma are sensitized to at least one indoor allergen.¹ This is especially relevant during the COVID-19 pandemic, when most are spending more time at home. There may be sensitive periods during a child’s life when aeroallergen sensitization may have a higher impact on health: a recent study demonstrated that sensitization at a younger age was associated with higher asthma risk later in life.¹ Unfortunately, health disparities also increase asthma risk. Children residing in lower-income urban areas have increased asthma severity, decreased asthma control, and thus higher implementation of urgent care or emergency visits.² This makes understanding indoor allergen exposure, especially in the inner-city, particularly important and an urgent public health matter.

In recent decades, salient indoor allergens have been identified, including dust mite (DM), cat, dog, mouse, cockroach and indoor molds. This review aims to summarize what is known about these allergens, and their effects on asthma control. Possible mitigation strategies and the evidence supporting them are also discussed (Figure 1). Furthermore, we review seminal studies as well as novel randomized controlled trials (RCTs) targeting allergen reduction strategies and their outcomes. To date, many intervention trials with expected positive outcomes have demonstrated mixed results (Table 1). PubMed was used to identify updated practice parameter guidelines on indoor allergen exposures, as well as seminal studies, and recent innovative peer-reviewed intervention trials. Search terms included “indoor allergen” “dust mite” “mouse allergen” “cat allergen” “dog allergen” “mold allergen” “allergen reduction intervention.” Further studies were selected based on relevant citations.

Figure 1:

Common indoor allergens and interventions for reduction of indoor allergen exposure.

Evidence level for effectiveness in reducing allergen noted in parenthesis following recommended intervention. Evidence level graded when available for the intervention according to the Scottish Intercollegiate Guidelines Network recommendations: Ia, evidence from meta-analysis of RCTs; Ib, evidence from at least one RCT; IIa, evidence from well-designed controlled study without randomization; IIb, evidence from at least one other type of well-designed quasi-experimental study; III Evidence from well-designed descriptive studies; IV, Evidence obtained from expert committee reports and/or clinical experience of respected authorities.⁶⁰

Table 1:

Select Randomized Controlled Trials Evaluating Reduction in Aeroallergen Exposures and Health Outcomes (2001–2021)

Reference (year)	Allergens Targeted	Population	Intervention	Trial Results: Allergen Outcomes	Trial Results: Health Outcomes
Carter (2001) 57	DM, cockroach	104 inner-city asthmatics aged 6–16 yo	Intervention group received allergen-impermeable covers, cockroach bait, education regarding dust mite (DM) and roach limitation. Placebo group received (sham) allergen permeable covers, instructions to wash in cold water. Control group received routine medical care	Significant reduction defined as 70% decrease, no difference found between intervention and placebo groups	Reduction in acute visits for asthma in intervention group Decreased acute visits in those with DM allergy who had decreased DM exposure
Morgan (2004) 10	Indoor allergens and tobacco smoke	937 inner-city asthmatics with aeroallergen sensitization aged 5–11 yo	Intervention group received 1 year of education, allergen-impermeable covers, HEPA vacuum and bedroom filter, personalized education and Integrated Pest Management (IPM) based on sensitization and exposure Control group evaluation every 6 months	Reduced DM, cockroach allergen levels	Reduction in urgent visits Decreased asthma symptom days during the intervention year, as well as the following year
Eggleston (2005) 58	Indoor allergens (cockroach, mouse) PM10 and PM2.5	100 low-income 6–12 yo asthmatics	Intervention group 1 year of education, allergen-impermeable covers, bedroom HEPA filter, IPM based on sensitization and exposure Control group treatment at end of 1 year	Approx. 50% reduction in cockroach allergen level, approx. 39% reduction in PM10 and PM2.5	Reduction in daytime asthma symptoms
Bryant-Stephens (2009) 59	DM, pests, pets, tobacco smoke	264 asthmatics 2–16 yo	Two intervention groups, one immediate and one delayed: education, allergen-impermeable covers, cockroach bait, mice traps, replacement carpet, curtains	Reductions were seen in pests, presence of carpets in bedrooms, and dust.	Nighttime wheezing significantly reduced after the intervention in both groups (P < .001). Mean number of emergency visits decreased by 30% after the intervention. Inpatient visits decreased by 53% (P < .001) after the intervention.
DiMango (2016) 46	DM, cockroach, mouse, cat, dog	110 asthmatic adults and 137 asthmatic children, all with aeroallergen sensitization	Asthma control optimized prior to randomization. Intervention group 40-wk education, allergen-impermeable covers, bedroom HEPA filter and HEPA vacuum, Control group education not including allergen avoidance	Intervention group displayed reduced DM, cockroach, mouse, cat, dog Control group: reduced cockroach, DM and mouse	Asthma control improved in both groups
Matsui (2017) 37	Mouse allergen	350 asthmatic children who were sensitized and exposed to mouse allergen	Intervention group Rodenticide, sealing portals of rodent entry, traps, allergen-impermeable covers, HEPA filter. If mice remained, further treatment to eliminate infestation Control group education regarding mechanisms to reduce mouse in the home	~70% reduction in mouse allergen in both groups, no difference in allergen levels between them	Reduction in mouse allergen associated with improvements in asthma symptoms, rescue inhaler use, urgent visits. No difference in symptoms between groups.
Rabito (2017) 45	Cockroach allergen	102 moderate-severe asthmatics aged 5–7 yo, homes cockroach-infested	Intervention group cockroach trapping and baiting at 0, 1, 3, 6, 9, and 12 mo. Control group cockroach trapping, but no baiting, at 0, 1, 3, 6, 9, and 12 mo.	Reduction in cockroaches in homes with trapping and baiting.	Reduced asthma symptoms and urgent health visits Less participants with FEV1 <80% predicted
Murray (2017) 11	DM allergen	286 DM sensitized asthmatics, 3–17 yo, with prior emergency hospital visits	Intervention group 12 mo. with allergen-impermeable covers for bed Control group 12 mo. with no allergen-impermeable covers	Mattress DM level reduced by 84% in the intervention group, no change in the placebo group (P< 0.001). Floor DM levels unchanged in both groups (P = 0.48).	Reduction in hospital visits for asthma exacerbation. No reduction in need for oral corticosteroids.
Phipatanakul (2021) 33	Mouse, rat, cat, dog, DM, mold allergens	236 asthmatic elementary students	Intervention group Schoolwide IPM (rodenticide, sealing entry, traps, cleaning, education). Additional treatments for infestation. Classroom HEPA filters. Control group No IPM, sham classroom HEPA filters	At baseline, 98% classrooms had detectable mouse allergen. More than 90% of baseline levels for cockroach, rat, Alternaria, Aspergillus, DM were undetectable. IPM did not significantly reduce any measured allergens. HEPA filters significantly reduced airborne mouse and dog allergen	Asthma symptom days decreased 0.4 days in the schools with IPM. After use of HEPA filters in classrooms, asthma symptom days were reduced 0.2 days. Neither produced a significant reduction in asthma symptom days overall. Post-hoc, IPM reduced asthma symptoms significantly by 63% early in the school year during fall/winter exacerbation season, but the benefit was not sustained

DM: dust mite, IPM: Integrated Pest Management, HEPA: High-efficiency particulate air filter

In this review, we also discuss the many complicating factors involved in studying the reduction of allergic exposures and measuring their impacts on health outcomes. Our intent is that this review serves to guide the clinician in advising his or her patients regarding evidence-based allergen reduction strategies, as well as guides the researcher in formulating future successful intervention trials that produce meaningful impacts on child health.

DUST MITES

It has been over half a century since DM was recognized as an important indoor allergen. Two varieties of DM are commonly discussed as relevant allergens, Dermatophagoides pteronyssinus and Dermatophagoides farinae. Eighty-four percent of US homes have detectable DM. Homes that are older, single family, carpeted and without air conditioning generally have higher levels of DM. Demographic factors also play a role; higher population density, lower household income, and co-existence of detectable cockroach and mold allergens are also factors associated with increased DM exposure.³ DM exposure is an unequivocal trigger in many atopic diseases, and for children sensitized to DM, exposure has been casually linked to asthma exacerbations.⁴

Given the detrimental effects on those sensitized, several evidence-based strategies are recommended to limit exposure to DM. Strategies aimed at reducing humidity to 35–50% are recommended. It is ideal to remove unnecessary carpeting, however, if carpeting cannot be removed, steam vapor treatment demonstrated the greatest DM population reduction (p < 0.05) when compared to vacuuming, or chemical methods of DM mitigation. Overall, the two physical methods, steam vapor and vacuuming, had higher efficacy when compared to chemical methods on reduction of DM (p = 0.002). Further, in a laboratory setting, the steam-vapor treatment effectively killed DM and denatured DM allergen.⁵

The bedroom is a primary focus of DM avoidance strategies.⁶ Removing extraneous fabrics and upholstery may eliminate sources of DM.³ Fine woven fabrics with a pore size <6–10 microns in diameter are reasonable as DM encasements for home furnishings such as mattresses and pillows that cannot be removed.⁷ Washing bedding regularly on a hot cycle and then drying in a dryer can kill DM, but this is best done every week.^{8, 9} HEPA room filters can trap particles as small as 0.1 mm, but also may disturb DM from the floor.⁶

Well-designed RCTs in children have demonstrated the efficacy of avoidance measures to reduce DM levels and some have successfully reduced complications of asthma.¹⁰ In an RCT published in 2017, Murray and colleagues studied the impact of allergen-impermeable covers on a cohort of DM-sensitized asthmatic children who had been previously hospitalized for asthma. In the intervention group, DM levels in dust from the children’s mattresses were reduced by 84%, with no change in the placebo group (P<0.001). DM levels from the floor were unchanged in both groups (P = 0.48). At 12 months, significantly fewer children in the group using allergen-impermeable covers presented to the hospital with an exacerbation. The risk of emergency department evaluation was 45% lower in the intervention group compared with the control group (hazard ratio, 0.55; 95% confidence interval [CI], 0.36–0.85; P = 0.006). However, there was no significant difference between the groups in the risk of prednisolone use for exacerbation (hazard ratio, 0.82; 95% CI, 0.58–1.17; P = 0.28).¹¹

Unfortunately, not all DM reduction trials have consistently improved health effects.¹² Difficulty arises as managing DM is not a “once and done” task, but an ongoing effort, and continuing mitigation measures are paramount to continued success.¹⁰

CAT

A National Health and Nutrition Examination Survey revealed that 50% of those questioned had a pet dog or cat, and 14% had both.¹³ Cat allergens are ubiquitous in society and are easily transferred. In fact, when homes with and without cats were studied, 95% of homes without cats had detectable Fel d 1, and airborne Fel d 1 was present in 25% of these homes.¹³ Although some families may intentionally avoid having certain pets at home due to a child’s allergies, it is important to understand that pet allergens are often carried into schools on the clothing, shoes, or belongings of other students. Higher environmental levels of cat allergen are associated with higher rates of sensitization and asthma symptoms among sensitized students.^{14, 15} Cat allergen challenges produced both clinical and biomarker responses in those sensitized, suggesting the detrimental impact of cat exposure on asthma morbidity.^{13, 16} As children spend a large proportion of their days outside their homes at school and daycare centers, aeroallergen exposures in these environments are critical to understand.

When there is a cat residing in the home, the most effective means of reducing exposure is to remove the animal.¹⁷ Once a cat is removed from the home, however, families must be cautioned that it may take up to six months to reduce cat allergen levels, so immediate improvement is not expected.¹⁸ Many families find removing the cat from the home an impossibility. Strategies to limit cat allergen exposure when the animal cannot be rehomed include keeping the animal out of the bedroom, and washing the animal at least twice weekly, however, effects are short-lived. Immersing the cat for 3 minutes removed Fel d 1 and reduced airborne Fel d 1 temporarily, but the benefits persisted for less than 1 week.¹⁹ Consistent vacuuming with HEPA vacuum, and using HEPA air filters have been studied as interventions in reducing detectable cat allergen in living room and bedroom samples. After 12 months of consistent vacuuming with HEPA attachment, participants had improved FEV₁ and peak expiratory flow compared with those who used a standard vacuum.²⁰ Similarly, HEPA filters have demonstrated efficacy in removing airborne cat allergen and improving airway hyperresponsiveness in asthmatic children who were sensitized to pets.²¹

Many cat owners affected by allergy find rehoming their animals, frequently bathing their animals and consistently vacuuming impractical. Other effective and practical interventions aimed at limiting the major cat allergen are needed. More recently, modifying a cat’s diet has shown promising results. A specific neutralizing antibody, anti-Fel d 1 IgY, has been added safely to cat’s food. Ingestion by the animal was associated with a reduction in allergen levels and minimization of the owner’s symptoms of allergic rhinitis.²²

DOG

Dog allergens have been associated with allergic rhinoconjunctivitis, asthma, and urticaria. In a recent population based study, 12.2% of the US population was sensitized to dog.¹³ However, sensitization to dog allergen does not always predict clinical signs of allergy.²³ There are several distinct dog allergens (Can f 1–7) detectable now with serum component testing. While many may be sensitized to dog IgE, Can f 1 shows the highest positive predictive value for clinical allergy to dog. Can f 5 is expressed in the prostate and secreted in the urine of male dogs. Component testing for dog allergens may confirm sensitization to related lipocalins or albumins of other animals.²⁴ Data for claims of “hypoallergenic dogs” is unsubstantiated as Can f 1 levels are similar between breeds considered “hypoallergenic” and those that are not.²⁵

As seen with cat exposure, dog allergen is detectable even in pet-free environments. Custovic et al evaluated the distribution of Can f 1 in homes with and without a dog and found levels above 10 micrograms/gram in 98% of homes with a dog and 16% of homes without a dog. Airborne Can f 1 was detectable in 30% of homes without a dog.²⁶ Outside of homes, one study of public buildings detected Can f 1 in 100% of dust samples.²⁷ Living with dogs may also reduce pulmonary function, purely by increasing endotoxin levels in a home.²⁸ Strategies for reducing dog allergen include rehoming the dog if possible, frequent bathing of the animal, vacuuming with HEPA-integral vacuum, and the use of HEPA filters.

MOUSE

Important mouse allergenic proteins include Mus m 1 and Mus m 2 from mouse urine, dander and hair follicles. Mouse allergen is ubiquitous, especially in inner-city homes, where studies have detected mouse allergen in 95%−100% of homes.^{29, 30} Mouse allergen is not only relevant to urban homes, but is also frequently detected in homes of suburban children with asthma.³¹ In a nationally-representative US survey, 82% of homes overall had detectable mouse allergen.³²

Although mouse allergen is prevalent in US homes, exposure to mouse allergen in schools is also common. In a 2021 study of classrooms in the inner-city northeast, 98% contained detectable mouse allergen.³³ In an earlier study, by Sheehan and colleagues, mouse allergen was detected in 99% of school dust samples, and levels of mouse allergen in schools were significantly higher than in students’ homes (median settled dust level, 0.90 vs 0.14 μg/g; P < .001).³⁴

Exposure to high levels of mouse allergen in schools is associated with increased asthma symptoms and reduced pulmonary function in children independent of allergic sensitization. Exposure to higher levels of mouse allergen in school (comparing 75th with 25th percentile) was associated with increased odds of having an asthma symptom day (OR = 1.27; 95% CI, 1.05–1.54; P = .02) and 4.0 percentage points lower predicted forced expiratory volume in 1 second (95% CI, −6.6 to −1.5; P = .002).³⁴ Independent of home exposure, school exposure to mouse allergen was linked in a dose-response manner to worsened asthma symptoms and diminished lung function.³⁵

To minimize mouse allergen exposure, integrated pest management (IPM) strategies should be employed, including removal of food sources for rodents, barring pathways of entry, and eliminating rodents already present through traps and rodenticides.³⁶

A 2017 study by Matsui and colleagues evaluated the effect of an IPM intervention on reduction of mouse allergen levels and also its effect on asthma morbidity among children sensitized to mouse. The Mouse Allergen and Asthma Intervention Trial randomized mouse-sensitized asthmatic children to pest management education with formal home IPM or pest management education alone. The IPM intervention included rodenticide, sealing external portals of entry, trap placement, targeted cleaning, installation of allergen proof covers and portable air purifiers. Mouse allergen levels decreased significantly in both groups, but there was no significant reduction in maximal asthma symptom days between the two groups. However, it was noted that clinical benefit increased with greater reduction in mouse allergen. A child experiencing a 90% reduction in bedroom floor mouse allergen levels was estimated to have 14.1 fewer maximal symptom days (P < .001), 16.4 fewer days of short-acting β-agonist use (P < .001), 16.7 fewer days of exercise-related symptoms (P < .001), and 0.8 less acute visits (P < .001) per year compared with baseline.³⁷ Reduction of mouse allergen may reduce future risk of deterioration in lung function: children who had 75% reduction in mouse allergen exposure had increased projected lung growth.³⁸

Meaningfully reducing mouse allergen levels enough to impact clinical outcomes may be complicated by housing characteristics. A subsequent analysis of the above study, published in 2021, evaluated baseline housing characteristics associated with successful mouse allergen reduction after intervention. In the intervention group, higher baseline bedroom floor mouse allergen levels were significantly associated with successful mouse allergen reduction. In the IPM-randomized group, detached or semidetached home, and absence of cockroach infestation were significantly associated with successful mouse allergen reduction. In the education group, higher percentage of clutter-free floor in the bedroom and absence of a basement were significantly associated with successful mouse allergen reduction.³⁹

COCKROACH

Bla g 1 and Bla g 2 are the primary roach allergens studied. In a sample of inner-city homes, cockroach allergen was detectable in 94% of kitchen floor samples.⁴⁰ Exposure to Bla g 1 levels greater than 1U/g is associated with increased sensitization to roach.^{40, 41} It follows then that sensitization to cockroach has been reported in as high as 60–80% of asthmatic children living in urban areas, while only 21% in a suburban population.^{41, 42} Sensitization and exposure to cockroach has been associated with increased hospitalizations and urgent medical evaluation for asthma,⁴³ as well as reduction in lung function.⁴⁴

The most effective intervention for cockroaches involves professional IPM, which can reduce exposure by up to 90%. Otherwise, strategies for reducing exposure are similar to reducing mouse, including removal of food sources, blocking mechanisms of entry, and eliminating cockroaches already present through traps and insecticides.⁴⁵

Studies examining reduction in cockroach allergen and associated asthma morbidity have demonstrated mixed results. On one hand, the Inner-City Asthma Study demonstrated that targeting cockroach and DM exposure reduced asthma symptoms during the 1-year intervention and this reduction persisted a year after the intervention ended.¹⁰ Another successful RCT by Rabito and colleagues, published in 2017, used a single intervention, insecticidal bait, to reduce cockroach exposure in the homes of children with asthma. Intervention homes had significantly fewer cockroaches than did control homes. Children in control homes had more asthma symptoms and unscheduled health care utilization in the previous 2 weeks (1.82, 95% CI, 0.14–3.50, P = .03; 1.17, 95% CI, 0.11–2.24, P = .03, respectively) and a higher proportion of children with FEV₁ of less than 80% predicted (odds ratio, 5.74; 95% CI, 1.60–20.57; P = .01) compared with children living in intervention homes. The authors stressed that insecticidal bait is a low-cost, and safe intervention that reduced cockroach exposure improved asthma outcomes.⁴⁵ In contrast, in another RCT of inner city adults and children, environmental control measures significantly decreased cockroach allergen levels, but these interventions did not significantly improve health outcomes.⁴⁶

INDOOR MOLDS

Fungi are ubiquitous in outdoor and indoor environments. They can enter indoor environments and then accumulate on surfaces, with growth occurring where there are sufficient nutrients and moisture. Fungal particles may be related to adverse health effects in those sensitized to mold, as well as those who are not sensitized but have underlying respiratory disease and are susceptible to irritant effects from mold in their environments. Indoor mold exposure may increase risk of asthma development and morbidity, even after controlling for outdoor fungal exposures.⁴⁷ A prospective cohort study followed 442 children through the first 6 years of life, and determined that exposure to mold in the bedroom, living room, or kitchen during infancy was associated with asthma development.⁴⁸ A meta-analysis and systematic review concluded that exposure to Penicillium, Aspergillus, and Cladosporium may increase the risk of developing asthma symptoms, while exposure to Penicillium, Aspergillus, Cladosporium, and Alternaria were associated with increased exacerbation of in those with current asthma symptoms.⁴⁹

For sensitized youngsters, exposure molds in daycare facilities is associated with wheezing. Chen et al. demonstrated that mold exposure in school classrooms was significantly associated with current asthma symptoms. These asthma symptoms then improved over holidays and weekends when the students were out of school,⁵⁰ similar to what is seen in adult occupational asthma.

In a prospective study, Baxi et. al. followed 280 children with asthma from 37 inner-city schools. Fungal spores were collected twice during the year, and were present in all classroom samples. Exposure to Alternaria was significantly associated with asthma symptom days in students sensitized to Alternaria (OR = 3.61, CI = 1.34–9.76, P = .01), but not in children not sensitized to Alternaria (OR = 1.04, CI = 0.72–1.49, P = .85). Baxi and colleagues also noted a dose-related increase in symptoms in sensitized students. Students sensitized to Alternaria and exposed to high levels of the fungus (≥75th percentile exposure) had 3.2 more symptom days per 2-week period compared with sensitized students exposed to lower levels.⁵¹

Interventions to reduce indoor mold exposure include reducing moisture, applying fungicides, and removing contaminated areas. An RCT of 62 asthmatic children living with indoor mold evaluated mold remediation versus information regarding home cleaning. There was a significant decrease in asthma symptom days after remediation compared with baseline, as well as decreased rates of exacerbation compared with the control group.⁵² However, a Cochrane Review including 12 studies and 8,028 participants, found moderate to very-low quality evidence that repairing mold-damaged homes and offices decreases asthma symptoms and respiratory infections in adults. Furthermore, there was very-low quality evidence that remediating schools for mold reduced student’s visits to physicians due to symptoms of the common cold.⁵³

There are several considerations as to why interventions targeting health outcomes related to reducing mold exposure have not been entirely successful. Many studies have been conducted in homes, when environmental interventions in schools have the potential to impact many students simultaneously, rather on a home by home basis and may improve childhood asthma. Further, in general, the impact of fungal exposure on health is nuanced. Diversity of molds in the environment as well as the timing of the exposure may alter outcomes. A recent study by Tischer et al. reported that exposure to higher fungal diversity in the perinatal period was linked to lower risk of wheezing and aeroallergen sensitization in later childhood.⁵⁴

CONCLUSION

Associations between indoor aeroallergen exposures and deleterious health outcomes among those sensitized, and in some cases even among those who are not sensitized but have underlying asthma, have been well-established. While we are beginning to understand how to mitigate these exposures, not all allergen reduction interventions have been successful in improving health outcomes.

Evaluating the reduction of indoor aeroallergen exposures and their effects on health outcomes is complex. While some studies did not effectively reduce the target allergen, others offered very aggressive allergen avoidance techniques in both the control as well as intervention arms such that additional interventions did not produce a significant change in the exposure. This complicates the evidence supporting or refuting individual environmental controls for allergen avoidance. Even after successfully reducing levels of the target allergen, studying the impact on health outcomes is complicated as many patients are sensitized to numerous allergens simultaneously. Community-based interventions targeting reduction in levels of multiple aeroallergens simultaneously in public settings may be more effective. Mitigation strategies for one allergen may not adequately address exposure to other relevant allergens. Some studies also may have enrolled patients with relatively controlled disease and low allergen exposure at baseline, thus any intervention was unlikely to effectively reduce exposure enough to meaningfully improve health.⁶

Asthma is a multifactorial disease, and other triggers besides aeroallergen exposures are known to exacerbate disease. Indoor air pollutants are known to impact asthma control, with exposure linked to increased rates of hospitalization, as well as ICU admissions.⁵⁵ We are beginning to understand that these pollutant exposures may modify the contribution of indoor aeroallergen exposures on health outcomes. A recently published secondary analysis of an RCT determined that environmental mouse allergen reduction was associated with improvements in asthma, especially among those with high baseline mouse allergen exposure. Lower indoor PM₁₀ was associated with greater improvements in asthma symptoms after mouse allergen reduction.⁵⁶

The contribution of viral infections on reduced asthma control is especially relevant in the pediatric population. We must appreciate the significance of the combined effects of allergic, irritant and viral triggers on asthma control. A recent blinded factorial RCT randomized 236 asthmatic students from 41 elementary schools and a total of 209 classrooms to a school-wide IPM program, use of HEPA filters in the classroom, or both. Control schools received no IPM, cleaning or education. Asthma symptom days over 2 weeks were studied, with a baseline mean of 2.2 days. After schoolwide IPM, asthma symptom days were reduced to 1.5 versus 1.9 days in the schools with no IPM. After use of HEPA filters in classrooms, asthma symptom days were reduced to 1.6, versus 1.8 in the classrooms with sham HEPA filters. While IPM and HEPA filters did reduce asthma symptom days, neither intervention overall produced significant reduction throughout the school year. However, in post-hoc analysis, students in IPM schools had a significant 63% reduction in asthma symptoms during the fall/winter season (typically a time of peak seasonal exacerbations), but the benefit was not sustained. Importantly, school absences were also significantly decreased in the school-wide IPM intervention. The authors postulate that combined reduction of both allergic and irritant triggers of asthma limited the susceptibility to viral-induced wheeze that is common during the fall/winter season.³³ The interplay between indoor allergens, irritants and viral infections complicate the study of the impact on indoor allergens on asthma morbidity. Multifaceted interventions reducing all of these triggers are more likely to be successful.

Designing a successful intervention trial may also involve evaluating predisposing genetic factors. Patient-specific genetics may mediate differences in asthma incidence and severity, even among patients with similar sensitization profiles and comparable aeroallergen exposures. Aside from genetic factors, diversity of microbial exposures in conjunction with environmental aeroallergen exposures may also play a role and complicate our understanding of health outcomes related to indoor aeroallergen exposures. Genetic polymorphisms and microbial exposures likely modify the impact of aeroallergen exposures on asthma control. Further delineating these interactions through carefully conceived, rigorous clinical trials, may be integral to our understanding of the impact of aeroallergen exposures on asthma, and warrant further study.

Key Messages.

• In recent decades, many indoor allergens have been identified, including dust mite (DM), cat, dog, mouse, cockroach and indoor molds, which have important health effects particularly in sensitized asthmatics.

• Sensitization to dog allergen does not always predict clinical signs of allergy. Serum component testing can identify sensitization to individual dog allergens, and Can f 1 shows the highest positive predictive value for clinical dog allergy.

• While pet allergens are likely transferred from homes rather than intrinsic to the schools themselves, other allergens, such as mice, mold and dust mites, are present in high levels in schools themselves and may have critical consequences to asthmatic students.

• Trials aiming to reduce allergen exposure and improve asthma control have had mixed results.

Abbreviations:

DM

dust mite

HEPA

high-efficiency particulate air filter

IPM

integrated pest management

SICAS-II

School Inner-City Asthma Study II