In the past several months, three schools in the San Diego region have revealed the presence of alarming levels of lead in their drinking water.

Lead is unsafe at any level and it is especially damaging to children’s brains.

Now, San Diego Unified and other school districts across the county are moving to test many more schools.

Here’s what we know and what we do not.

Three Schools Reported Lead Problems

At La Mirada Elementary School in San Ysidro, samples taken last fall found alarming levels of lead coming from eight drinking fountains.

At the shared campus of Emerson-Bandini Elementary School and the San Diego Cooperative Charter School 2 in Mountain View, tests taken in early March found alarming levels of lead at one water fountain and two sinks.

At San Marcos Middle School, a test of a drinking fountain near the gym had “higher than acceptable” levels of lead, the district revealed in early March.

It is possible that more schools will be added to this list because San Diego Unified, the largest school system in the county, is planning to test all its schools in coming months. Other school districts are also getting their campuses tested.

Before those tests results come back, Assemblywoman Lorena Gonzalez advised parents to send their kids to school with bottled water, if those schools were built before 1986 – the year Congress passed a law to ban lead plumbing.

“We know that lead poisoning is irreversible,” she said in a written message to us. “We don’t know the extent of the issue. So, as a parent, I would err on the side of caution until we know more.”

Brent Eidson, a spokesman for the city water department, said San Diego “prides itself on the safety and quality of the water we provide to customers,” but deferred comment to San Diego Unified. The school system says on its website that it is working to have every school in the district tested before the start of summer. It also plans to post the results of those tests on its website.

The Water Supply Is Not the Problem, Plumbing Is

The problems at the schools so far seem to be with pipes and fixtures at the schools themselves. That means the overall drinking water supply is safe.

We know this a few different ways.

First, some sinks, faucets and fountains in these three schools did have high lead levels. In fact, water samples from those same schools lacked any detectable amount of lead at all. That suggests that the water picked up lead as it touched specific pipes or fixtures inside the schools.

We know even more about the water going to La Mirada and the shared Emerson-Bandini/charter school campus, which both receive water from the city of San Diego’s water department.

Last year, the city did routine tests of water leaving three of its drinking water treatment plants. None of those tests indicated high lead levels, according to data posted on the city of San Diego website.

In February, after concern erupted about lead in San Ysidro, the city took additional samples of water in its system. The samples were taken near to the schools but before the water entered them. If there was any lead in the water before it reached those schools, the amount was so low that it could not be detected in the city’s laboratory. The lab can detect lead at levels less than one part per billion. The maximum level of lead allowed in drinking water is 15 parts per billion.

There’s a Lot We Don’t Know

That may not offer much assurance, though, because people don’t drink water as it leaves a water treatment plant, nor do they drink water out of the city’s water lines.

People drink water that comes from sinks and fountains and after it has winded through largely untested plumbing.

Water departments know very little about what water is like inside a school, church, office, home or apartment.

That said, water departments are required to do a few tests for lead in a few homes. But those tests are extremely limited.

Even in cities with a half million housing units, like San Diego, the water department only must test, at most, 100 homes a year.

Because San Diego’s tests of homes have not shown problems in the past, it now only must test 50 homes every three years.

During the last testing cycle, in 2014, the city tested 54 homes. It also did a special round of tests in 2016 of 21 homes to see if water from the new desalination plant in Carlsbad was chemically different enough from the city’s traditional water supply to cause pipes to corrode and release metal into the water.

All told, only three sites in 2014 had detectable levels of lead and only one of those, a site in Old Town, was above the level considered alarming by water quality regulators. During the 2016 cycle, the city found no cause for concern because of the desalinated water.

Oddly, of the 93 tests for lead done over the past four years, 29 were done at locations in the 92111 zip code, which is the Linda Vista area.

Eidson, the water department spokesman, said that lead testing focuses on homes built in the mid-1980s, years which are considered high risk. The reason for that narrow window appears to be that lead was banned after 1986 and newly-installed pipes were the most likely to leach lead into the water. That means pipes from the early-1980s were of concern at the time.

Once the city started testing for lead in homes in the early-1990s, it had to keep testing the same homes or find homes nearby, Eidson said.

For whatever reason, Linda Vista is now being tested a lot, while homes in San Ysidro and some other populous or low-income neighborhoods are not being tested at all. The other neighborhoods to have frequent tests are Tierrasanta, Old Town, Rancho Bernardo, Scripps Ranch and Del Mar. No other ZIP codes were tested more than five times in the past four years.

Meanwhile, La Mirada and Emerson-Bandini are in neighborhoods that have not been tested at all.

There is some indication parents there should be worried about lead generally, though lead comes from places other than water – it can come from old paint, for instances. Such old paint is more likely to be chipping away from old buildings in low-income neighborhoods, like the ones where those two schools are.

Both neighborhoods have higher-than-average amounts of lead showing up in children’s blood, according a national analysis of childhood blood tests by Reuters.

You can search your neighborhood using this amazing map database Reuters created (scroll down the linked page to see the map).