Snap on the lights at Janet Brown's North Philadelphia house and see the roaches scatter.

Though they run like a retreating army, they're rarely beaten, living well in walls and small spaces, often moving easily from one home to the next in low-income neighborhoods with endless blocks of aging rowhouses.

For Brown and her son, Ronnie, 11, exposure to roaches means asthma.

"Last year, he was hospitalized six times," Brown said. "It wears me out."

When you're poor, Brown knows, you live with vermin, an extra layer of low-income misery.

More than 26 percent of Philadelphia children in a recent study were found to have asthma, nearly three times the national average and among the highest rates in America. The biggest factor contributing to asthma in children in the urban Northeast is exposure to roaches - mostly in poor neighborhoods, experts say.

Ironically, the chemicals that people use to kill roaches and rats - pests that are also connected to asthma - exacerbate their asthma.

Some compounds do more harm than that.

Tres Pasitos, a foreign rat poison illegal in the United States because it is highly toxic, is used by some Latinos in the city, potentially endangering the lives of children who sometimes ingest it, experts say. It means three little steps - after eating it, that's as far as rats can go.

"All of this is stressful to parents," said Hernando Perez, an environmental health scientist with Drexel University's School of Public Health. "And in Philadelphia, these problems come together in low-income environments."

Scientists can't directly say roaches cause asthma.

But they know that the more people are exposed to the feces and shedded skins of cockroaches, the greater the chance that they'll develop asthma, according to pediatrician Tyra Bryant-Stephens, director of the Community Asthma Prevention Program at Children's Hospital of Philadelphia. Such people are believed to have allergies to roaches. Bryant-Stephens led the Philadelphia asthma study, which was published in the Journal of Asthma in 2012.

The city is trying to reduce asthma. Using $1 million in federal funding, the Department of Public Health recently instituted the Healthy Homes Healthy Kids program to educate families about asthma and help eliminate pests.

In Philadelphia and elsewhere, minority children suffer disproportionately from asthma.

Nationwide, about 13 percent of black children have it, compared with 8 percent of whites and Latinos, according to the U.S. Environmental Protection Agency.

But Puerto Rican children in particular have the highest rates of asthma at 20 percent, according to the EPA.

Many black and Latino children may have a gene that makes them more susceptible to asthma, scientists say.

Black children are twice as likely to be hospitalized for asthma and four times as likely to die from asthma as white children, since access to care can be more limited for low-income children, the EPA said.

While asthma is rarely deadly, two or three children died from it in Philadelphia last year, said Michelle Niedermeier, program coordinator with the Pennsylvania Integrated Pest Management Program. She said reporting was imprecise. Her program, at Pennsylvania State University, studies ways to eliminate causes of pest problems.

A recent study in the Journal of Allergy and Clinical Immunology shows that early exposure to air pollution, along with a genetic component among minorities, may increase the risk of cockroach allergy that causes asthma in young children, said the study's author, Matthew Perzanowski, a Columbia University scientist.

Along with roaches, people in poor neighborhoods are plagued with mice and rats, themselves connected to asthma.

As a result, some children eat or put their mouths on rat poisons in the home, according to Kevin Osterhoudt, director of CHOP's Poison Control Center.

Last year, 73 Philadelphia children 6 and younger were reported to have been exposed to such poisons, he said. None died. Many more cases occur than are reported, Osterhoudt said.

Between 8,000 and 13,000 such child exposures were reported nationwide. Few if any died, primarily because most U.S. rat poisons aren't strong enough to hurt humans.

But in poor Spanish neighborhoods in Philadelphia, Tres Pasitos is being seen more frequently, according to Chris Haines, director of emergency medicine at St. Christopher's Hospital for Children.

Often sold in flea markets as loose pellets in unmarked baggies, Tres Pasitos contains aldicarb, one of the most toxic pesticides, according to the EPA. Aldicarb can paralyze the respiratory system and kill a person, the EPA said.

Haines once treated a girl who ingested the poison. Her breathing accelerated and fluid filled her lungs. Fortunately, she recovered, Haines added.

Acute Tres Pasitos cases are rare, Osterhoudt said. But he, added, the compound "is out there, and is very dangerous."