In August, doctors diagnosed a beloved Philadelphia teacher with mesothelioma, a deadly cancer caused by asbestos. She was exposed to asbestos during her nearly 30-year career at two elementary schools in South Philadelphia.

Then, late last month, school district officials halted a multimillion-dollar construction project in a building that housed nearly 1,000 students from two schools: Benjamin Franklin High School and Science Leadership Academy in the city’s Spring Garden section. The district was forced to cancel classes there after its environmental staff and its hired consultants dropped the ball and failed to flag all asbestos materials that were present before the start of construction. That failure resulted in work crews exposing damaged asbestos and unknowingly causing contamination.

On Monday, Superintendent William R. Hite Jr. plans to hold two town hall meetings at district headquarters designed to assuage fears and answer questions from parents and teachers. He also will discuss options for relocating Ben Franklin and SLA students to other schools as construction progresses and until the building is deemed safe for their return.

Here’s a primer on what parents need to know about asbestos in schools and answers to questions about the district’s responsibility to protect students. Let’s start with the basics:

What is asbestos and why is it in my child’s school?

Asbestos is a naturally occurring, fibrous mineral that was once prized for its durability and heat resistance. It can be found all over schools built before 1980 — wrapped around steam pipes and duct work as insulation, sprayed and troweled onto walls and ceilings as soundproofing in auditoriums and woven into floor and ceiling tiles. Even the adhesive that holds floor tiles in place is likely to contain asbestos. A 2018 Inquirer investigation, “Toxic City: Sick Schools,” documented roughly 11 million square feet of asbestos that remains in district buildings. That’s enough to cover the Philadelphia Convention Center more than 11 times over.

» READ MORE: Read more: The Inquirer's investigation into asbestos perils in Philadelphia schools

So why doesn’t the school district remove it all?

If undamaged and in good condition, asbestos is not considered a health hazard. In fact, removing large amounts of asbestos in schools could create more of a risk, especially if not done properly. Removing it is also very expensive and time-intensive. Specially trained asbestos crews must don protective suits and respirators, isolate and “tent” the abatement area with thick plastic sheeting, install air filtration, wet the material to keep the dust under control, set up a decontamination chamber and showers for workers going in and out of the space, and lastly, take air samples during the job to make sure fibers haven’t escaped outside the abatement area and retest the air after the work is done.

How does asbestos get damaged and why is that a concern?

Years of wear and tear can cause asbestos materials to break down and release fibers into the air. When old heating pipes leak, for example, the asbestos insulation becomes wet and then dries, with a repeating cycle that cracks the material and releases fibers. Chipped or damaged floor tiles are a concern because of the volume of kids walking or running over them each day. The Inquirer investigation found that more than 80 percent of district schools had damaged asbestos and the district struggled to stay on top of it.

Who’s at risk and why?

Building engineers and school custodians are most at risk because they spend more time in areas with lots of asbestos, like boiler rooms, and because they are sweeping up dust, dirt and debris that might contain asbestos fibers. Teachers ranked third — behind plumbers, mechanical engineers and electricians — for professions at higher-than-normal risk of getting mesothelioma, according to the National Institute for Occupational Safety and Health (NIOSH). Mesothelioma, an aggressive cancer with a zero survival rate, is relatively rare. It has a long latency period and it can take 10 to 50 years or longer for symptoms to appear. Asbestos also can cause asbestosis, a debilitating lung disease. And, medical experts say that asbestos exposure can increase the risk of a variety of other cancers, including of the lungs, larynx, esophagus, stomach, kidney, colon, rectum and ovaries.

What about children? Are they vulnerable?

Yes. Marilyn Howarth, a physician at the Center of Excellence in Environmental Toxicology at the University of Pennsylvania, said in an interview last month that “children are more at risk for exposure because they tend to breath through their mouth so they bring the air through their trachea and not through their nose, where it might get caught in mucus.” She said children also run around more than adults, “which makes them breathe faster ... and their lungs are still developing.”

How do I know whether my child’s school contains damaged asbestos and where?

The Inquirer last year created “School Checkup,” an easy-to-use web tool that pulls together internal district reports on incidents of damage to asbestos and other health hazards in nearly 200 schools. Parents can plug in the name of a school and learn of hazards, by exact location and type, down to the classroom level. Also, under federal law, the district is required — every three years — to do a comprehensive building inspection for each school, noting the location of asbestos material and whether it’s damaged. The district also must do a visual walk-through of each school every six months. You can find the 2015-16 school year reports here. Over the summer, the district completed several inspections for the 2018-19 school year but many have yet to be published online. You are legally entitled to view those on demand and the district is required to keep copies for parents to view in each school’s office.

The district officials say highly trained environmental crews perform some 200 abatement jobs each year and air samples taken after work completion yielded “negative” results for asbestos fibers? Does that mean the air is safe?

It depends. For the majority of abatement jobs, the district relies on a method of air testing for asbestos using an old microscope technique that is too crude to detect the thinnest and shortest asbestos fibers, said Jerry Roseman, the environmental science director for the union that represents teachers. It is only in the case of really major abatement jobs that city health regulations dictate that once the work is done, the district is required to use a much more sophisticated and costly type of air testing, a high-magnification method.