The findings suggest that "the real costs of this pollution are substantial," said lead researcher Sylvia J. Brandt, of the University of Massachusetts, Amherst.

Similar cost studies could be done in other U.S. communities, Brandt told Reuters Health.

Researchers estimate that the annual cost of traffic-related asthma symptoms in the two cities -- Long Beach and Riverside -- totals about $18 million. That's considering the direct costs of medical care, as well as indirect expenses like kids' school absences and parents' time off from work.

The figure is an estimate, based on government cost data and past research on the number of childhood asthma cases linked to traffic pollution.

And other communities' costs would vary widely depending on, among other things, the amount of traffic near homes and schools.

As an example, Brandt said that dozens of communities in Massachusetts have higher asthma rates than those seen in the two California cities -- where an estimated 13 percent to 15 percent of kids younger than 18 have asthma.

Springfield, for one, has an asthma prevalence of 18 percent, and it's surrounded by major highways, she noted.

Nationally, it's estimated that 9 percent of U.S. children have asthma. But rates are higher among poor, minority kids; 16 percent of low-income black children have asthma, according to the U.S. Centers for Disease Control and Prevention.

There are a number of risk factors for asthma, including genetic vulnerability. But, Brandt pointed out, there's growing evidence that traffic pollution may not only trigger symptoms in kids who already have asthma, but may also boost the odds of developing asthma.

A number of studies have found that the closer a child lives to heavily traveled roads, the higher the asthma risk -- even when controlling for other factors like family income.

COULD NEW ZONING HELP?

For the current study, reported in the European Respiratory Journal, Brandt's team used estimates from an earlier study that linked road traffic to 9 percent of all childhood asthma cases in Long Beach and 6 percent in Riverside.

They calculated that over a year, the costs of asthma caused by road pollution, and asthma attacks triggered by pollution, would reach about $18 million for the two communities.

Brandt acknowledged that there is "uncertainty" in estimating such costs.

But she said that it would be useful for communities to have an idea of their own figures so they can see where their money is being spent, and how it could be better used.

Based on her team's estimates, Riverside spends 6 percent of the county's health-and-welfare budget on pollution-related asthma; the costs in Long Beach, meanwhile, equal about 21 percent of the city's health department spending.

"The findings are at least suggestive that other communities could have high costs as well," Brandt said.

Cities and towns with pollution issues could take steps to cut children's risk of breathing problems, Brandt said -- like passing laws against building schools or residential developments close to highways.

And in schools that are already near high-traffic roadways, she added, "clean air" technologies might help.

SOURCE: bit.ly/z3UtTs European Respiratory Journal, online January 20, 2012.

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