Association between subway iron particulate matter exposure and respiratory disease in New York City.

Particulate matter exposure is linked to increased morbidity and mortality. Iron-rich particulate matter (PM2.5), common in rapid transit systems, is a potential but understudied contributor to respiratory illness. Using electronic health records (EHR) from 452,272 patients in the INSIGHT Clinical Research Network in New York City (2020-2023), we examined whether local iron exposure is associated with asthma, chronic obstructive pulmonary disease (COPD), breathing difficulties, or respiratory inhaler use. Iron exposure was estimated using particulate matter measurements from New York City (NYC) subway stations, linked to each patients residential census block group. To account for potential non-linear relationships, we applied linear probability models and an adjacent block group estimator with paired fixed effects to assess respiratory outcomes across deciles of iron exposure. We found that the relative risk of developing asthma, COPD, or breathing difficulties increased by 6-15% between the lowest two exposure deciles. Beyond this range, there was no significant association between iron exposure and respiratory disease. This suggests that iron exposure from rapid transit is associated with respiratory disease primarily at lower exposure levels, with limited health benefits from marginal reductions in iron exposure at already high exposure levels.