It came too late. Dean lost his ability to do simple math, such as adding three plus three in his head, calculations that once came easily to him, his parents said.

“When you send your child to school, you think he’s going to be safe and you don’t have to worry,” his father, David Pagan, said.

Every school day in Philadelphia, children are exposed to a stew of environmental hazards, both visible and invisible, that can rob them of a healthy place to learn and thrive. Too often, the district knows of the perils but downplays them to parents.

As part of its “Toxic City” series, the Inquirer and Daily News investigated the physical conditions at district-run schools. Reporters examined five years of internal maintenance logs and building records, and interviewed 120 teachers, nurses, parents, students, and experts.

When the newspapers analyzed the district records, they identified more than 9,000 environmental problems since September 2015. They reveal filthy schools and unsafe conditions — mold, deteriorated asbestos, and acres of flaking and peeling paint likely containing lead — that put children at risk.

PHILADELPHIA FEDERATION OF TEACHERS HEALTH & WELFARE FUND Mold in Kindergarten classroom A-2 at Farrell Elementary.

Even so, the district often takes months, even years, to complete repairs, the investigation found.

To independently determine potential health hazards facing students, reporters enlisted staffers at some of the district’s most rundown elementary schools to conduct scientific tests for toxins.

In all, staffers at 19 schools, following testing guidelines, used surface wipes to sample areas for lead dust, mold spores, and asbestos fibers. They also collected water from drinking fountains. The newspapers used a nationally accredited lab to analyze the samples.

A disturbing picture emerged.

Dangerously high levels of cancer-causing asbestos fibers were found on surfaces in classrooms, gymnasiums, auditoriums, and busy hallways — residue from crumbling pipe insulation, damaged floor tiles, and deteriorating ceilings.

Tests revealed lead dust, at hazardous levels, on windowsills, floors, and shelves in classrooms, including one for children with autism.

School district officials took issue with the newspapers’ testing. They questioned the collection methods, and argued that air monitoring for asbestos — not dust wipes — is far more accurate and the only testing method required by law.

Dangerous Lead Dust in Schools Of 14 elementary schools tested for lead by the newspapers, 10 had unsafe levels of toxic residue from flaking paint on windowsills, shelves or classroom floors. The Environmental Protection Agency considers lead a hazard if there is more than 40 micrograms of lead per square foot on floors, and more than 250 micrograms per square foot on windowsills. SOURCE: Samples independently obtained by Philadelphia Media Network; Analysis conducted by International Asbestos Testing Laboratories GARLAND POTTS / Staff

As for building conditions, district officials said they are saddled with too many old, deteriorating schools and too little money. It will take $3 billion over the next 10 years to build new schools, replace roofs and heating systems, and finish all urgent repairs, they said.

School officials also cite their inability to fill vacancies in custodial and maintenance staff and hire enough tradespeople to maintain the buildings. (The district recently started a program in which graduates of its vocational high schools will be hired as apprentice plumbers, electricians and HVAC technicians.)

The district spent $69 million in fiscal 2017 on cleaning, maintenance and renovations.

Eileen Duffey, a longtime school nurse, said it would only take "a reasonable amount of money" to prevent children like Dean from getting sick. Families are required by law to send their children to school, she noted, so the district at a minimum should provide a healthy environment.

"We have to figure out how to get out of the mess we're in," she said. "And in no place is it more important to provide that environment than in the schools where we require them to spend the greatest portion of their lives."

Lower standards for schools

Philadelphia children are at particularly high risk for lead poisoning because more than 92 percent of the city’s homes were built before 1978, the year the federal government outlawed residential use of lead paint to protect people from “the unreasonable risk of injury.”

Young children are most susceptible to the ravages of lead, a potent neurotoxin that can permanently damage a child’s brain, reduce IQ, and cause behavioral problems. Whether inhaled in airborne dust or ingested, the heavy metal is unsafe even in minuscule amounts.

At least three-quarters of children in Philadelphia district schools are from low-income families. Poor children tend to have poor nutrition, lacking calcium and iron, making it easier for lead to be absorbed in the body. Once absorbed in bones, lead stays for life.

In 2012, concerned Philadelphia lawmakers made it illegal for landlords to rent homes with damaged lead paint to families with children ages 6 or younger.

Yet that same law doesn’t apply to the city’s schools, even though 90 percent of them were built before the 1978 ban. Many of the district’s 130,000 Philadelphia children spent their school hours in buildings with damaged lead paint.

No city, state, or federal laws require that district schools be free of lead hazards.

Since the district does not routinely test classroom floors and surfaces for lead contamination, reporters had staffers at 14 elementary schools follow an EPA-approved protocol to collect surface dust for testing. The results were startling:

A sample from a second-grade classroom floor at A.S. Jenks Elementary School in South Philadelphia tested at 1,100 micrograms per square foot — almost 28 times the federal hazard level for residential floors.

A sample from a second-floor hallway at J. Hampton Moore Elementary School, in Northeast Philadelphia, tested at 1,400 micrograms of lead per square foot — 35 times higher than the federal hazard level for residential floors. In the ceiling directly above was a gaping hole, since fixed.

One dust wipe sample from the floor of a classroom for autistic children at Olney Elementary School tested at 5,900 micrograms per square foot — almost 150 times the federal hazard level.

And at Henry H. Houston Elementary School in Mount Airy, one dust sample taken from a second-floor hallway tested even higher, 6,300 micrograms per square foot.