WASHINGTON, Feb. 13 -- Exposure to air pollution during pregnancy may increase the mom-to-be's risk of developing high blood pressure, a new U.S. study said Thursday.

Researchers at the University of Florida found that exposure to four air pollutants, including two specific types of fine and coarse particulate matters, sulfur dioxide and carbon monoxide, may be just as toxic to pregnant women, if not more so, as breathing in cigarette smoke.

According to the U.S. Environmental Protection Agency (EPA), particulate matters include acids, dust, metals and soil particles, which are released from industries and forest fires and can form when gases react with each other in the air. Sulfur dioxide is emitted from power plants and industries while most carbon monoxide is produced by car exhaust.

"Fetal development is very sensitive to environmental factors," said Xiaohui Xu, an assistant professor of epidemiology in the university.

"That is why we wanted to do this research. Hypertension (high blood pressure), in particular, is associated with increased morbidity and mortality, causing a lot of problems for the mother and fetus, including preterm delivery."

The researchers examined data from 22,000 pregnant women who gave birth between 2004 and 2005 in Jacksonville, Florida, and then gauged how much pollution the women were exposed to throughout their pregnancies using data the EPA gathered daily.

Mothers who suffered from chronic hypertension, had previously given birth prematurely and had complications during delivery were excluded from the study.

The researchers found 4.7 percent of women in the study developed hypertension during pregnancy.

After controlling for other risk factors such as socioeconomic status, exposure to smoking, the researchers concluded that exposure to air pollutants throughout the first two trimesters of pregnancy increased women's risk of developing one of these conditions.

But they could not determine conclusively whether exposure early in the pregnancy or late in the pregnancy was more likely to increase a woman's risk for hypertension.

"It looks like the whole period has impacts for hypertension," Xu said.

The findings were published in the Journal of Epidemiology & Community Health.