The wafers appear innocuous and have not until recently been at the top of many hazardous material lists. But a key chemical in many, paradichlorobenzene, can trigger asthma attacks and is a suspected carcinogen.

Following the lead of several other government agencies, including Erie County, the state Department of Correctional Services, and the cities of New York and San Francisco, the state Legislature this year voted to ban the cakes, also known as para blocks, from public and private school bathrooms. The ban could take effect as soon as November, depending on which version of the legislation Gov. George Pataki signs. While a spokeswoman for Pataki said he has not yet received the bill, one of the main sponsors, Assemblyman Michael Gianaris, D-Queens, said the governor had said he supported the move.

"We thought the place we could have the most impact was the schools," said Gianaris, who didn't rule out future efforts to broaden the ban.

The way in which this ban came about shows how toxic substances can remain relatively obscure until some unexpected event focuses attention on them.

Concern over para blocks came to the forefront in New York last year when an unusually high number of cancer cases among firefighters based in a Brooklyn firehouse was linked to the para block fumes.

Environmentalists hailed the ban.

"I'm delighted to think that somebody is going to ban these chemical toilet bowl cleaners," said Claire Barnett, executive director of the Healthy Schools Network, an Albany-based group that pushes to remove toxins and other pollutants from schools.

"This is a trend," added Cameron S. Lory, a research associate at Inform, a New York City group that advises governments and businesses on how to reduce pollution.

Lory noted that there are substitutes for the para blocks, including wafers that use bacteria to contain odors. And newer auto-flush urinals eliminate much of the need for such cakes, she said.

The substitutes, however, are costlier, even though they last three times as long, added Lory.

Representatives from Willert Home Products Inc. of St. Louis, a major para block manufacturer, didn't return calls for comment.

Environmentalists such as Barnett and Lory note that even benign deodorizers pose their own dangers, in that they mask what could be the need for better cleaning and disinfecting of restrooms. "They actually affect your sense of smell," Lory said of deodorizers.