Air pollution with smaller particulate matter had stronger associations with childhood asthma than pollution with larger particles, indicating that smaller particles may be more toxic, according to a study published in JAMA Network Open.

Efforts to purify the air, control particulate matter pollution and develop air quality guidelines could reduce the adverse effects of this pollution, especially for children, Chuansha Wu, PhD, of the department of environmental hygiene and occupational medicine at School of Public Health, Medical College, Wuhan University of Science and Technology, and colleagues wrote.

Caregivers of 29,418 children (52.1% boys; mean age, 4.9 years; standard deviation [SD], 0.9; age range, 3 to 6 years) in seven cities in China responded to a questionnaire and indicated that 3.9% of these children had been diagnosed with asthma, and 8.6% had experienced wheeze.

Also, 62.9% were breastfed for more than 6 months, 2.7% had a parental history of atopy, 2.3% had a mother who was a current or former smoker during pregnancy and 29.7% had passive household cigarette smoke exposure in early life.

The researchers also used a mature machine learning-based method to estimate daily mean concentrations of ambient particulate matter (PM) in these cities between January 2013 and December 2018.

Mean early-life exposures for these children included:

• 36.7 µg/m³ (SD, 8.9) for PM₁, or PM with an aerodynamic equivalent of 1 µm or less;
• 20.7 µg/m³ (SD, 4.6) for PM_1-2.5, or PM between 1 µm and 2.5 µm;
• 61.7 µg/m³ (SD, 13.1) for PM_2.5, or PM of 2.5 µm;
• 48.9 µg/m³ (SD, 16.6) PM_2.5-10, or PM between 2.5 µm and 10 µm; and
• 110.6 µg/m³ (SD, 19.3) PM₁₀, or PM of 10 µm.

Each 10 µg/m³ increase in early-life PM₁ exposure was associated with a 55% increase in risk for childhood asthma (OR = 1.55; 95% CI, 1.27-1.89). Each 10 µg/m³ increase in early-life PM_2.5 exposure (OR = 1.14; 95% CI, 1.03-1.26) and in early-life PM₁₀ exposure (OR = 1.11; 95% CI, 1.02-1.2) also increased risk for childhood asthma.

However, the researchers did not find any association between asthma and exposure to PM_1-2.5 or PM_2.5-10, indicating that PM₁ and not PM_1-2.5 contributed to the association between PM_2.5 and childhood asthma.

The researchers further found associations between risk for childhood wheeze and each 10 µg/m³ increase in early-life PM₁ (OR = 1.23; 95% CI, 1.07-1.41) and PM_2.5 (OR = 1.08; 95% CI, 1.01-1.16) exposures, although there were no associations between wheeze and other size-segregated particles.

There were increased risks for childhood asthma with each 10 µg/m³ increase in prenatal (OR = 1.29; 95% CI, 1.12-1.5) and first-year (OR = 1.52; 95% CI, 1.25-1.84) exposure to PM₁ as well. First-year exposure to PM₁ also had a greater risk for childhood wheeze (OR = 1.2; 95% CI, 1.05-1.37) than prenatal exposure to PM₁ (OR = 1.14; 95% CI, 1.03-1.26).

Although they concluded that there were statistically significant associations between early-life PM₁ exposure and elevated risks for childhood asthma and wheeze, the researchers noted that there is limited epidemiological evidence of an association between PM₁ and respiratory diseases.

Still, the researchers continued, there was an association between PM₁ exposure and respiratory toxic effects, indicating an urgent need for more studies to explore the adverse effects of PM at this size on human health.

Air pollution with smaller particles has greater association with childhood asthma