Chicago ? Pesticide use in or near U.S. schools sickened more than 2,500 children and school employees over a five-year period, and though most illnesses were mild, their numbers have increased, a nationwide report found.

Sources include chemicals to kill insects and weeds on school grounds, disinfectants, and farming pesticides that drift over nearby schools, according to the report by researchers at the National Institute for Occupational Safety and Health and their colleagues.

Lead author Dr. Walter Alarcon said one of the largest recent incidents occurred in May when about 600 students and staff members were evacuated from an Edinburg, Texas, elementary school after pesticides sprayed on a cotton field drifted into the school’s air conditioning system. About 30 students and nine staffers developed mild symptoms including nausea and headaches.

The study, which appears in Wednesday’s Journal of the American Medical Association, covered events from 1998 to 2002 – none as big as the Texas incident, Alarcon said.

Activists seeking to reduce pesticide use contend many commonly used pesticides, including some involved in the study incidents, can increase risks for cancer, birth defects and nerve damage.

“The chronic long-term impacts of pesticide exposures have not been comprehensively evaluated; therefore, the potential for chronic health effects from pesticide exposures at schools should not be dismissed,” the authors wrote.

Still, the overall rate of pesticide illnesses in schools is small – 7.4 cases per million children and 27.3 cases per million school employees, the authors said.

Jay Vroom, president of CropLife America, which represents suppliers of farming pesticides, said the report is alarmist and that pesticide use around schools “is well-regulated and can be managed to a level that does not present an unreasonable health risk.”

Allen James, president of RISE, a trade group for makers of pesticides used in schools, faulted the study for relying on unverified reports and said the numbers nonetheless suggest that incidents are “extremely rare.”

The authors tallied reports from three pesticide surveillance systems, including a national database of calls to poison control centers and found that 2,593 students and school employees developed pesticide-related illnesses in the five years studied. Only three illnesses were considered severe.

Most of the illnesses were in children. The number of children affected each year climbed from 59 to 104 among preschoolers and from 225 to 333 among children age 6 to 17.

“I don’t think we want to overwhelm people, but the study does provide evidence that using pesticides at schools is not innocuous and that there are better ways to use pesticides,” said study co-author Dr. Geoffrey Calvert.

Claire Barnett, of the Healthy Schools Network advocacy group, said the total is likely a “deep undercount” because there are about 54 million U.S. schoolchildren and yet no comprehensive national tracking system.

The authors said the study underscores the need to reduce pesticide use through pest management programs that typically require schools to use pesticides as a last resort, and to implement advance written notification when the chemicals are used. The guidelines also often recommend that spraying in schools or in nearby fields should occur only when students and staffers are not present.

Laws in 17 states recommend or require schools to have such programs, according to Jay Feldman, executive director of the Beyond Pesticides advocacy group.