High Ambient Temperature and Preterm Birth in California, 1999-2006

RUPA BASU (1), Brian Malig (1)

(1) California Office of Environmental Hazard Assessment

     Abstract Number: 10
     Last modified: August 25, 2009

Abstract
It is essential to tease apart the effects of either ambient temperature and air pollution and health outcomes using epidemiologic methods. Previous studies have reported associations between air pollution and adverse birth outcomes. However, the relationships between temperature/heat and adverse birth outcomes, such as preterm delivery, have yet to be studied.

The objective of this study was to estimate the effect of apparent temperature on preterm delivery independent of air pollutants. To focus on heat effects, our study was limited to the warmer months, May to September 1999-2006.

We conducted a case-crossover analysis of almost 60,000 births spanning 16 Californian counties. Subjects were identified from California state birth records, which were combined with meteorological and air pollution data.

An 8.6% (95% confidence interval: 6.0, 11.3) increase in preterm birth per 10 degree Fahrenheit increase in mean weekly apparent temperature was found. The association was not found to be confounded or modified by ozone, particulate matter, carbon monoxide, nitrogen dioxide, or sulfur dioxide. The best model fit was observed for mean apparent temperature in the prior week. High ambient temperature had a significant impact on preterm birth for all mothers, regardless of racial/ethnic group, maternal age, maternal education, or sex of the infant. The highest effects were observed for younger mothers, and Blacks and Asians.

Higher ambient temperature is associated with substantial increased risk of preterm delivery for all subgroups considered. Thus, preventive measures should be considered for all mothers, especially those who are at greater risk for preterm delivery during the summer months. Although this is the first large population-based study on temperature and preterm birth to find a positive association, further study is warranted given the relatively mild climate in California.