LAUSR

RATIONALE Ambient ultrafine particles (UFPs; <0.1 µm) may exert greater toxicity, compared to other pollution components, due to enhanced oxidative capacity and ability to translocate systemically. Studies examining associations between prenatal UFP exposure and childhood asthma remain sparse. OBJECTIVES We used daily UFP exposure estimates to identify susceptible windows of prenatal UFP exposure with asthma in children, accounting for sex-specific effects. METHODS Analyses included 376 mother-child dyads followed since pregnancy. Daily UFP exposure during pregnancy was estimated using a spatiotemporally-resolved particle number concentration prediction model. Bayesian distributed lag interaction models (BDLIMs) were used to identify sensitive windows for UFP exposure, and examine whether effect estimates varied by sex. Incident asthma was determined at first report of asthma (3.6+3.2 years). Covariates included maternal age, education, race, and obesity, child sex, nitrogen dioxide (NO2) and temperature averaged over gestation, and postnatal UFP exposure. MEASUREMENTS AND MAIN RESULTS Women were 37.8% Black and 43.9% Hispanic with 52.9% reporting