In the city’s poorest neighborhoods, people may be exacerbating the very disease they are trying to overcome–asthma–by buying industrial strength toxic pesticides.

The pesticides, which are sold on the street but are meant to be used only by licensed exterminators, do kill the rats and roaches that asthmatics are often allergic to. But what buyers don’t realize is that they contain chemicals that can aggravate the disease–and may even be fatal.

The roach and rodent killers are sold by street vendors in the Bronx, the Lower East Side, Washington Heights and other Spanish-speaking neighborhoods under names like Tempo and Tres Pacitos.

It was a packet of Tempo, a brownish powder sold in little foil packets, that tipped off Dr. Stephen Frantz, an asthma researcher from the state Department of Health, that people trying to fight asthma by killing rats and roaches might be exposing themselves to harmful chemicals.

“Someone came into an apartment where I was doing my asthma work and handed me a packet of this stuff,” he said. He stuck the envelope in his pocket and forgot about it. Later he realized the resident shouldn’t have had access to the product, which was supposed to be highly diluted and handled only by licensed exterminators. Professionals know that the product is supposed to be diluted–but it doesn’t say so on the packet.

He also discovered the pesticides contained cyfluthrin, a synthetic form of pyrethrin, a class of natural pesticides that have been shown to worsen asthma. At low concentrations, pyrethrins have been known to induce mild coughing, sneezing and asthmatic attacks. Some people’s faces swell up after exposure. At higher concentrations, pyrethrins can cause bronchial spasms and even death.

Tres Pacitos comes in granules that residents typically mix with bait to lure rats and mice. The name, people say in the Bronx, is Spanish for what a poisoned rat has time to take before it dies: three steps. Frantz notes that Tres Pacitos contains aldicarb, one of the most potent neurotoxic pesticides available in the United States. Meant only for agricultural use, aldicarb is not registered for legal use in New York.

Bronx resident Amy Rosado sprinkled Tempo behind her stove and along her floorboards. She stopped using it after it gave her itchy, watery eyes and sneezing fits. “Our neighborhoods before were so filled with roaches, we were just trying to clean,” said Rosado, who works for a nonprofit now educating residents about the hazards of these pesticides. She tells her neighbors not to use the chemicals, but she still sees people selling and buying them in her neighborhood. “It’s still going strong. I guess because people are still using it.”