WASHINGTON (Reuters) - Toddlers who breathe polluted air are far more likely to be diagnosed with bronchitis than children living in cleaner environments, U.S. and Czech researchers reported on Thursday.

They found a component of pollution known as polycyclic aromatic hydrocarbons, or PAHs, was strongly linked with cases of bronchitis among children aged 2 to 4 and a half.

The study is one of the first to look at PAHs, which are produced when fuels that contain carbon such as wood, coal, diesel or tobacco are burned.

Most environmental regulations in the United States and Europe focus on controlling particulate emissions -- tiny particles in the air -- as well as sulfur dioxide and ozone.

"Our work strongly suggests that regulators consider efforts to curb PAHs as well," Irva Hertz-Picciotto of the University of California, Davis, who led the study, said in a statement.

Hertz-Picciotto and colleagues examined the medical records of 1,133 children up to age 4.5 born in two districts of the Czech Republic between 1994 and 1998. One, Teplice, has high levels of air pollution while the other, Prachatice, has less.

As in many other areas of Europe and in the United States, cases of bronchitis rise in the winter and are often more common where people burn wood in stoves or fires. The researchers collected detailed air quality data, with rare samplings of PAHs, which are difficult and expensive to measure. The Czech government and the U.S. Environmental Protection Agency helped pay for the study.

"It is an impressive amount of very complete data, giving us a rare opportunity to look at the impact of air pollution on the health of young children and the actual components comprising that pollution," said Hertz-Picciotto.

"We were able to correct for everything from duration of breast feeding to smoking in the home to average daily temperature."

Writing in the journal Environmental Health Perspectives, the researchers said they found cases of bronchitis were 56 percent higher in toddlers when PAH levels were high.

They found a 29 percent increase for children under 2.

"We found that polycyclic aromatic hydrocarbons have a significant effect on the lung health of children in the study, in particular in increases in acute bronchitis diagnoses for toddlers and preschoolers. We saw the biggest impact on children old enough to play outside, while infants were affected, but not quite as much," said Hertz-Picciotto.

Her team does not know why PAHs might cause or worsen bronchitis but said they may cause inflammation.