Traffic pollution, particularly in urban areas, exacerbates symptoms of asthma in children suffering from the respiratory illness, Mexican researchers said in a study published Thursday.

The researchers found that this worsening of symptoms meant the children had to use medication more often.

The Mexican research team, led by Dr. Isabelle Romieu of the Institute Nacional de Sauld Publica in Cuernavaca, Mexico, said an increase in the levels of atmospheric pollutants led to an increase in coughing, wheezing and use of medication among asthmatic children, while it led only to an increase in coughing among healthy children.

The study, published in the online journal Respiratory Research, suggests that small buses for public transportation running on gasoline or natural gas and larger buses and trucks running on diesel fuel were more strongly associated with the worsening of symptoms.

Researchers urge government to act

The researchers said the findings make it clear that the Mexican government should take action to curb traffic pollution.

"Given the important adverse effects of these traffic exhaust pollutants, greater, stringent control of vehicular emission and traffic is required, especially close to schools and in areas where children participate in outdoor activities," the study says.

"These results have significant public health policy implications, as a large proportion of schools in Mexico City and other countries are located very close to roads carrying heavy traffic."

The study followed 147 asthmatic and 50 healthy children for an average of 22 weeks in a southeastern area of Mexico City, where there is heavy truck traffic. The children, ranging in age from six to 14, were recruited from a pediatric hospital.

Parents of the study participants kept daily diaries to record the presence or absence of coughing, wheezing and breathing difficulty. In the case of the asthmatic children, parents also recorded the incidence of medication use.

The researchers recorded atmospheric levels of pollutants, namely ozone, nitrogen dioxide and diesel particles, at the public schools attended by the children in Mexico City during the course of the study.

They also noted the amount and type of traffic in areas inhabited by the children to find out whether vehicles powered by diesel had a greater impact on respiratory health than pollution from other vehicles. The researchers, who carried out the study between 2003 and 2005, then analyzed the data.

Results consistent with previous studies

"In this study, we observed that asthmatic children living in urban areas with high traffic density had a greater daily incidence of both respiratory symptoms and bronchodilator use," the study says. "Our results are consistent with those of previous studies."

The study notes that the Mexico City area is one of the largest and most polluted urban areas in the world and about 85 per cent of air pollution in the area is said to come from motor vehicles.

The researchers said they undertook the study because no specific data existed on the impact of traffic exhaust on the respiratory health of asthmatic and healthy children in Mexico City.

According to the researchers, there is still uncertainty about which type of vehicle emission causes the worst effects on health in children.