Exposure assessment. Hazardous air pollutant
(HAP) concentrations were assessed
by the U.S. EPA National Air Toxics
Assessments in 1990, 1996, 1999, and 2002,
which uses an inventory of outdoor sources
of air pollution, including both stationary
sources (e.g., waste incinerators, small businesses)
and mobile sources (e.g., traffic) to
estimate average ambient concentrations of
pollutants for each census tract based on dispersion
models (U.S. EPA 2011). Data were
downloaded from the U.S. EPA website on
23 June 2010; additional archived data was
received on compact disc from the U.S. EPA.
Air pollution concentrations were linked to
nurses’ residential locations at the time of the
birth of their child through census tract codes
(Neighborhood Change Database 2013).
Children were assigned pollution concentrations
from the U.S. EPA assessment closest
to their year of birth (births 1987–1993 used
1990 concentrations; births 1994–1997 used
1996 concentrations; births 1998–2000 used
1999 concentrations; births 2001–2002 used
2002 concentrations). We categorized each
pollutant according to quintiles of concentration
in the entire study population