Outdoor Environment and Pediatric Asthma: An Update on the Evidence from North America

Table 2

Association between specific pollutants and pediatric asthma.

Source/year	Outdoor variables	Age group	Sample size	Climate region	Study design	Assessment method	Findings and limits
Asthma diagnosis and symptom
Akinbami et al., 2010	SO2, NO, O3, PM2.5, PM10	3–17 years	34,073	National, US	Cross-sectional/adjusted	National Health Interview Survey (NHIS) database; stratified multistage sampling	aORs for current asthma for the highest quartile of estimated ozone exposure: 1.56 (95% CI: 1.15, 2.10) and for recent asthma attack 1.38 (95% CI: 0.99, 1.91). Limit: county-level 12-month averages of pollution are imprecise measures of children’s exposure to pollution.
Berhane et al., 2014	NO2, PM10, PM2.5, O3	5–7 years	1,211	Western, US	Cohort/adjusted	Questionnaire, FeNO measurement, ambient air monitoring stations	Increases in annual concentrations of 24-hr average NO2 and PM2.5 were associated with increase in FeNO. Limit: lack of information on time-activity patterns for the subjects could lead to misclassification of exposure
Cornell et al., 2012	BC, PM2.5		240	Northeast, US	Cross-sectional/adjusted	FeNO test, portable air sampling units, fixed BC monitor, PFT, Serum IgE	BC higher in high-asthma neighborhoods (1.59 µg/m^3 [95% CI 1.45–1.73]) than in low-asthma neighborhoods (1.16 µg/m^3 [1.06–1.27]) with . Limit: sample was limited to middle-income households
Ebisu et al., 2011	Urban land use, TRAP modeling, NO2	0-1 years	680	Northeast, US	Cross-sectional/adjusted	Interview, asthma diary	10% increase in urban land-use within 1,540 m buffer of infant’s residence associated with 1.09-fold increase in wheeze severity. This link becames insignificant with TRAP modeling added. Limit: NO2 as an indicator of overall TRAP misses other pollutants
Habre et al., 2014	PM2.5, O3,	6–14 years	36	Northeast, US	Cohort/adjusted	Symptoms diary, skin test, air sampling, air monitoring, temperature, and humidity	2 of the 3 highest frequency reactions were for ragweed (48%) and birch (39%). Exposure to O3 and particular matters was significantly associated with severe wheezing. Limit: reliance on central-site ambient measurements to assign outdoor exposure category
Jerschow, 2015	Dichlorophenols (pesticide)	≥6 years	2,125 sample (% of children unclear)	National	Cross-sectional/adjusted	NHANES, dichlorophenols measured in urine	Higher dichlorophenol levels were linked with asthma diagnosis, asthma prescriptions, missing work/school, exercise-induced wheezing in atopic wheezers. No association between dichlorophenol levels and asthma morbidity in nonatopic wheezers. Limit: reliance on self-reported data about wheezing problems.
Jung et al., 2012	Polycyclic aromatic hydrocarbons (PAH)	5-6 years	354	Northeast, US	Cohort/adjusted	Questionnaires, PAH air monitoring devices, blood samples	Repeated high exposure to pyrene was associated with report of asthma. Limit: PAH exposure was assessed only by 2 repeated measures 5 to 6 years apart, which could lead to misclassification
Lewis et al., 2013	PM10, PM2.5, O3	5–12 years	298	Upper Midwest, US	Cohort/adjusted	Respiratory symptom diary, ambient air monitoring, caregiver interview	Outdoor PM2.5, PM10, and O3 concentrations were associated with increased odds of respiratory symptoms, particularly in children using steroid medication. Similar associations were not realized with PM10-2.5. Limit: measuring symptoms using handwritten diaries by caregiver and the child could lead to errors.
Miller et al., 2010	Polycyclic aromatic hydrocarbons (PAH)	≥5 years	222	Northeast, US	Cohort/adjusted	Questionnaires, urine testing, immunoglobulintesting	Widely varying levels of 10 PAH urinary metabolites were detected in all children. Levels of PAH metabolites were not associated with respiratory symptoms. Limit: the half-lives of PAH metabolites are short and thus variations in exposure across time may be large.
Nishimura et al., 2013	O3, NO2, SO2, PM10, PM2.5	8–20 years	4,320	South, Northeast, West, Upper Midwest, Puerto Rico, US	Cohort/adjusted	Questionnaires, regional ambient air pollution data,	Early life exposure to NO2 was associated with risk for asthma [OR = 1.17; 95% CI 1.04–1.31] in Latino and African American children across 5 US regions. Other pollutants’ impact varied across regions. Limit: measurement of PM2.5 was less complete than that of other pollutants, leading to a smaller sample.
Padula et al., 2015	PAH	9–18 years	467	West, US	Cross-sectional/adjusted	PFTs, spirometry, skin testing, fixed air monitoring, wind and humidity	Significant association between PAH and lung function testing in nonasthmatic children: increase in PAH456 was associated with decrease in FEV1. Limit: change in pulmonary function over time wasn’t assessed
Patel et al., 2013	O3, PM10, PM2.5, NO2, , BC	14–19 years	36	Northeast, US	Cross-sectional/adjusted	Aethalometers to measure BC, EPA systems database, R-Tube, immunosorbent assays	BC and NO2 were positively associated with airway inflammation and oxidative stress. Limit: the use of central-site PM2.5 and O3 measurements could bias the effect estimate from them toward null.
Perez et al., 2012	NO2, O3	<18 years	2.54M +	Western, US	Cross-sectional/adjusted	ACS, local surveys, EPA air quality system, ambient air monitors, proximity to traffic	8% of asthma cases were partially caused by resident proximity to major road. Link between proximity to major road and asthma exacerbations is positive. Limit: traffic density and vehicular emissions are not reflected in this metric of traffic proximity
Pongracic et al., 2010	Fungal allergen exposure	5–11 years	936 children (moderate-severe asthma)	National, US	Cohort/Adjusted for covariates	Interviews, portable air sampling, site inspections, dust samples	Excess symptom days per 2 weeks associated with increase in outdoor fungi level; increases in total fungal exposure was associated with increases in symptom days and asthma-related unscheduled visits. Limit: the study did not have children not sensitized to fungal allergens
Ratnapradipa et al., 2013	Soot, exhaust, wood or oil smoke	<5-6 (pre-school)	691	Northeast, US	Cross-sectional/adjusted	Structured interviews	Exposure to soot, exhaust, wood, or oil smoke was associated with higher risk of asthma than those never exposed. Limit: the cross-sectional nature of the study and the recall bias were associated with interview-based data
Sarnat et al., 2012	BC, PM, , PM2.5,	6–12 years	58	South, US, Mexico	Cross-sectional/adjusted	eNO testing, air monitoring, air monitors, passive badge samplers, BMI measurement	There exists significant link between eNO and measures of PM and BC. PM pollutant levels predict acute respiratory responses better than NO2 measurements. Limit: clinical significance of the estimated increases in eNO with pollutant levels as observed here is unclear.
Spira-Cohen et al., 2011	PM2.5, SO2, Elemental carbon (EC)	10–12 years	40	Northeast, US	Cohort/adjusted	Questionnaires, air monitoring, time-activity daily diary, aethalometer, spirometry	Elevated risk of wheeze, shortness of breath, and total symptoms were associated with same-day increased personal EC, but not with personal PM2.5 mass. No associations with school-site PM2.5 or, SO2. Limit: a small sample size of only 40 study participant
Vette et al., 2013	PM2.5, BC, NO2, NOx, CO, , VOCs	14–16 years	139	Midwest, US	Cohort/adjusted	FeNO testing, nasal lavage, F2-isoprostances, air monitoring, diaries, air monitoring	This paper is a protocol, yet preliminary data provide evidence of roadway impacts on the measured concentrations and indicate that variations in exposures between study participants are evident. Limit: full detailed results are yet to come, not in this paper
Zora et al., 2013	, PM2.5, PM10, markers for TRAP (BC, NO2)	6–11 years	36	South, US	Cross-sectional/adjusted	Questionnaire, ambient air monitoring, meteorology data, pulmonary function testing	Positive (but not statistically significant) association between asthma and each single pollutant. Limit: use the questionnaire-based data as outcome variable could bring in recall bias, social desirability bias, etc.
Asthma cost
Brandt et al., 2012	NO2, O3	0–17 years	1,290	Western, US	Cross-sectional/adjusted	MEPS, CHIS, NHTS, HCUP, published averages of NO2 and O3	Nearly 50% is due to regional air pollution-attributable exacerbations among children with asthma. Limit: costs are usually difficult to measure
Asthma-related symptoms and care utilization
Strickland et al., 2010	, O3, NO2, SO2, CO, as markers for TRAP	5–17 years	91,386	Southeast, US	Cross-sectional/adjusted	Administrative data (ICD-9) from ED visits, ambient air quality monitors, pollen counts	Asthma ED visits associated with O3 during warm season and cold season (Nov–Apr), several TRAP measures in warm season, PM2.5 and SO2 in warm season, in cold season; associations with ED visits present at relatively low ambient concentrations of studied variables. Limit: difficult to draw causal inference from cross-sectional design
Tse et al., 2015	Wildfire exposure		2,195, 3,965	West, US	Cross-sectional/adjusted	Short-acting β-agonist (SABA) use in obese children	SABA use increased (+16%, ) in obese children (BMI > 30) compared to nonobese (BMI < 30) in 2003; increased but nonsignificant difference (+10.5%, N.S.) in SABA use in 2007. Limit: asthmatic patients may have taken preventive action to minimize the exposure
Lemke et al., 2014	NO2, SO2, VOC, PM10, PAH	5–89 years	2,900	Upper Midwest, US & Canada	Cross-sectional/adjusted	Geospatial data, air sampling station data, ICD-9 codes with ED visits and hospitalizations	Intraurban air quality variations related to adverse respiratory events; NO2, PM10, and VOC positively correlated with ED visits. Limit: relatively coarse temporal resolution in study design compromises generalizability
Evans et al., 2014	PM2.5, CO, SO2, O3	3–10 years	74	Northeast, US	Cross-sectional/adjusted	Physician visits, ER visits	Increases in UFP and CO concentration were associated with pediatric asthma visits. Increases in O3 were associated with less asthma visits. No associations for mode particles, BC, fine particles, or SO2. Limit: the monitoring station is located on a diesel bus route, which could lead to higher measured pollutant concentrations than the actual exposure among some of the study subjects.
Delfino et al., 2014	CO, N, PM2.5, O3, as markers for TRAP	0–18 years	11,390 visits/7,492 patients	Western, US	Cross-sectional/adjusted	Emergency Department visits, inpatient admissions; ambient air station data	ED visits and admissions for asthma were positively associated with ambient air pollution (i.e., O3, PM2.5) during the warm season, and CO, NO2, PM2.5 in the cool season. Limit: insurance status is the only individual-level sociodemographic information

BC: black carbon; ED/ER: emergency department/emergency room; eNO: exhaled nitric oxide; SABA: Short-Acting Beta-Agonists; UFP: ultrafine particles; VOC: volatile organic compound. Note. (a) estimated exposure levels using LUR modeling.