Philadelphia ? Doctors were awaiting test results Saturday from several suburban schools where students broke out in red, itchy rashes.

Dr. Norman L. Sykes, a Thomas Jefferson University dermatologist, said authorities are waiting for results of tests for fifth disease, a common ailment that causes a mild rash. Tests of one student and one teacher were negative, he said.

Investigators are looking into whether environmental factors, such as cleaning agents or some form of allergen, could have caused the rashes that have struck more than 100 students since Jan. 31.

They are also checking whether the schools have common sources of food or other schools that have reported rashes, Sykes said.

The rashes have been reported in at least eight schools some 20 miles apart in the suburbs 35 miles north of Philadelphia.

“We are emphasizing that they don’t have any reason to believe anything worrisome is happening,” Sykes said. Even if something in the schools did cause the rashes, he said, “It’s just an annoying, nagging itch.”

Health officials have tested water and inspected for chemicals without finding anything unusual.

Similar probes failed to find the cause of itchy rashes that affected hundreds of students and teachers in November at Marsteller Middle School in Manassas, Va.

In New York City’s Queens borough, 46 elementary school students and three teachers at PS 161 reported severe itching on Tuesday. The cause of the rashes remained unknown.

Rashes also broke out last week among several dozen students at Abraham Lincoln Elementary School in Medford, Ore.

The ailment cropped up recently in eastern Pennsylvania. Over the past eight weeks, 62 students at Tobyhanna Elementary Center in the Pocono Mountain School District have been diagnosed with fifth disease.

So called because it was once considered one of the five main childhood illnesses, fifth disease is usually mild, according to the National Center for Infectious Diseases. One in five people infected don’t develop symptoms.

A child may have a low fever or cold-like symptoms, followed by a face rash creating a “slapped cheek” appearance and a lacy red rash on the trunk, arms and legs. Adults may develop joint pain.

Sykes said the virus is spread by coughs and sneezes but the contagious stage is over by the time the rash appears. The rash usually disappears in seven to 10 days, and those infected develop lifelong immunity. Sixty percent of adults have been infected at some point in their lives.

Infection during pregnancy may cause serious complications, and fifth disease can slow red blood cell production and cause serious anemia in people with sickle-cell disease. There is no treatment other than to treat any fever, joint pain or discomfort from the rash.