Table 5.

Indoor Environment Effect on Childhood Asthma

Indoor air pollutants	Source, Effect and Recommended Intervention
Ambient air pollutants Particulate matter (PM) PM10 (diameter < 10 um, entering respiratory tract) Fine PM2.5 (diameter < 2.5um, entering alveoli)	Indoor sources: cooking exhaust, wood burning stoves/fireplaces, re-suspended particles from cleaning Increased airway reactivity, asthma exacerbations, respiratory symptoms, altered host defense(76–79) Decreased lung function and increased respiratory symptoms in children(80–82), provoke lung inflammation Exposure to high levels PM2.5 in children with asthma-associated with elevated FeNO(83) Causal link between exposure to PM and the development of asthma has not been established, but such a relationship may exist: children exposed to outdoor PM in utero or in the first year of life were at increased risk of having asthma at age 3–4 years(84) Intervention: high-efficiency particulate air (HEPA) filters- lowers the concentration of indoor PM; modest effect on reducing asthma symptoms(85–87)
Nitrogen dioxide (NO2)	Indoor sources: gas stoves, space heaters, furnaces, fireplaces Children exposed to ambient NO2 in utero or in the first year of life had an increased prevalence of asthma in early childhood(84) Exposure to indoor NO2 was linked to asthma exacerbation(88–91) and increased respiratory symptoms in children with asthma The association between NO2 and increased respiratory symptoms was stronger for non-atopic asthmatics than for atopic asthmatics(89) Pathophysiology: NO2 may cause respiratory symptoms by producing extracellular reactive oxygen species that interact with antioxidants in the lung epithelial layer(92, 93): increased respiratory symptoms in patients with allergic asthma Intervention(94): mean indoor NO2 was decreased by over 60% through the replacement of unflued heaters with flued heaters, with a concordant decrease in asthma symptoms in children
Second hand smoke (SHS)	There is a causal relationship between second hand smoking exposure and asthma incidence and morbidity in children(25, 95–99) SHS exposure in utero through maternal smoking was linked to decreased lung function, recurrent wheeze, and increased incidence of asthma in infants(100–103) In-utero smoke exposure was associated with an increased risk of corticosteroid resistance among adolescents with asthma(104) Intervention to reduce SHS exposure for children had limited success(105, 106). However, a recent study in Scotland (107) demonstrated a reduction in the rate of asthma related hospitalizations for children by 18.2% per year, following a smoking ban in public places in Scotland(107)
Indoor allergens	Effect and Recommended Intervention
Mouse allergen	Source: urinary allergens excreted and carried on airborne particles(108) Most inner-city and suburban homes have detectable mouse allergen levels(109–113) 18–50% of inner-city youth are sensitized to mouse allergen(113, 114), less common in rural/suburban(112, 115) Mouse sensitization and exposure in children with asthma is associated with increased hospitalizations, increased respiratory symptoms, decreased lung function, and increased airways inflammation(114, 116) Sensitization to mouse allergen has been causally related to early wheeze in on study(117): mouse allergy could increase the risk of developing asthma Intervention: integrated pest management (IPM) (extermination, cleaning, food disposal, sealing of cracks) A reduction in mouse allergen levels by at least 50% has been associated with reductions in missed school and days with decreased activity among mouse-sensitized inner-city children(116)
Cockroach	Source: at least half of inner-city homes have clinically relevant levels of cockroach allergen, 30% of suburban, middle class homes have detectable levels of cockroach allergen(118) In inner-city, 60–80% of children with asthma are sensitized to cockroach(119, 120) Sensitization to cockroach allergen has been linked to the development of wheeze in young children(117), poorer asthma outcomes in inner-city children with asthma and increased asthma related healthcare utilization(120, 121) Intervention: IPM reduced cockroach allergen levels by 80–90%(122) The effect of this reduction on asthma morbidity is unknown; but the strong link between cockroach allergen exposure and asthma morbidity supports recommending an IPM approach to reduce exposure
Cat or dog	Source: the allergen was found in all homes; concentrations were 10–1000 fold lower in homes without pets(123, 124). Allergic sensitization to cat and dog is common Pet allergen exposure has been linked to poorer asthma outcomes in sensitized patients with asthma(125, 126) Some studies suggested that pet exposure may be protective against allergy(127–129) Indoor pet exposure for children was associated with lower total IgE levels at age 18 years among atopic patients only(130), indicating that the protective effect of early life pet exposure that was observed at 6–7 years(129) was mostly lost by 18 years In contrast to these studies, a recent study in Norway(131) found that cat allergen exposure during early life led to an increased risk of asthma at age 10 years, but not to an increased risk of animal sensitization Intervention: further studies are needed to determine the role that early life pet allergen exposure may play in the development of asthma: until that time, there is not sufficient evidence to recommend early exposure to or removal of pets from the homes of children at risk for the development of asthma
Dust mites	Source: allergic sensitization depends on the humidity of the climate Dust mite allergen exposure has been causally linked with the development of asthma in susceptible children(26) and with asthma exacerbations in sensitized individuals Intervention: the 2007 NAEPP guidelines recommend installation of allergen-proof mattress and pillow encasements, washing of all bedding every 1–2 weeks in hot water, removal of stuffed toys, reduction of indoor humidity, and regular vacuuming and dusting. These strategies reduce asthma symptoms, medication requirements, and improve lung function(132, 133) 2008 meta-analysis(134): dust mite control measures do not improve clinical outcomes in patients with dust mite allergy. Platts-Mills(135) suggested that due to variability in methods among studies the current practices of dust mite control should not be changed
Molds	Allergic responses to inhaled mold allergens are associated with increased asthma symptoms Alternaria, found indoors and outdoors is associated with increases in asthma symptoms, bronchial hyper-responsiveness, and severe asthma in sensitized individuals(136–138) Exposure to high levels of indoor Penicillium was associated with increases in symptoms in children with asthma(139) Sensitization to mold was associated with increased frequency of asthma symptoms Poor asthma control in children has been linked to the presence of visible mold at home(140) Intervention: reduction of mold levels may be beneficial in controlling asthma symptoms Removing molds and eliminating leaks and moisture sources at homes to help reduce asthma symptoms(141)