Asthma cases that were caused or made worse by traffic pollution cost a total of $18 million a year on average in two California communities, according to a new study.

The total annual cost associated with a case of asthma was approximately 7-8 percent of average household income in Long Beach and Riverside.

Both communities have high levels of air pollution due to major highways running close to residential areas.

"Our findings suggest the cost has been substantially underestimated and steps must be taken to reduce the burden of traffic-related pollution,” said Sylvia Brandt, lead author of the study.

Brandt assessed the typical volume of health services and number of school absences for a child with asthma each year to come up with the annual cost estimates.

The results are published in the European Respiratory Journal.