In general, scientists found that emergency room visits rose along with ground-level ozone and certain pollutants directly released from cars.
The pollutants include carbon monoxide and certain types of fine particles small enough to be inhaled deeply into the lungs.
Experts say that ground-level ozone forms when sunlight reacts with pollutants from cars, factories and other sources.
Ground-level ozone is the main component of smog.
The link was stronger during warmer summer months.
The latest findings define which air pollution components are involved.
The study appears in the Journal of Respiratory and Critical Care Medicine.