How lead can get into the water supply, explained in 5 charts

Flint, Michigan, is not far from fresh water. The city is sandwiched between three of the Great Lakes: Lake Michigan about 140 miles west, Lake Huron just 40 miles north, and Lake Erie 100 miles or so south.

How did a city with such plentiful water sources end up with a terrible lead poisoning problem? The answer is, in part, infrastructure.

Water does not naturally contain much lead. But untreated water can damage lead pipes, causing the toxic element to seep into a water source. It can then pick up lead as it travels elsewhere. This is what happened in Flint: Untreated water from the Flint River moved through lead pipes, picking up the toxin as it went, and spread it throughout the city.

Lead in drinking water is not the main risk of lead poisoning — flaking of old lead paint is the most common cause. And in Flint there were factors well beyond lead pipes that contributed to the crisis continuing unnoticed for so long. Vox's Libby Nelson wrote an excellent explainer on the issue here.

Still, lead pipes aren't an issue specific to Flint: An estimated 10 million Americans get drinking water from pipes that are at least partially lead. These graphics explain how lead can get into drinking water — and why that can be a huge public health problem, as it is in Flint.

1) Lead is incredibly damaging to the body — and any exposure is a problem

The reason we worry so much about lead in water, in paint, or in any environment is that lead is extremely poisonous when ingested. Studies show it can damage the brain and kidneys and lead to higher risk for cancer and stroke. Lead accumulates in the body and builds over time, so ongoing exposure, even at low levels, can become toxic.

Children's growing bodies absorb lead more quickly and efficiently than adults. Lead can cause seizures, hearing loss, behavioral problems, brain damage, learning disabilities, and lower IQ levels in children.

The US Environmental Protection Agency says that lead is the most serious environmental health hazard for children under 6 years old in the United States.

2) Lead pipes can leach toxins into a water source

Lead occurs naturally in many places, but usually only in trace amounts. There may be some lead in reservoirs or rivers that supply drinking water, but not at high enough levels to be considered a threat to health.

On the other hand, parts of pipe networks can contain lead — and lead can get into water flowing through those pipes if they become corroded over time and through heavy use.

Water is naturally corrosive — its oxygen molecule will break down lead piping. Then the lead will seep into the water source.

If utilities don’t carefully balance water chemistry and treatment methods, tainted water can enter a community water system — like what happened in Flint.

3) The main risk of lead in drinking water comes from old service lines

Main water lines — made of iron or steel — are mostly safe. Cities and states have done a good job making sure that these pipes, which send water to large geographic areas, are now lead-free. But there are millions of lead service lines connecting older homes to water mains, especially in the Northeast and Midwest. If the pipes are exposed to corrosive water, or if water sits too long inside them, the lead could be released and may end up coming out of the tap.

Ownership of service lines is typically shared between homeowners and local water departments. The homeowner usually owns the section of pipe under the property — and in older homes, it is possible that no one has done the work to switch out lead pipes.

4) In some old houses, lead can seep into a water source from old plumbing

The service lines bring water from a larger source to a home. Within older houses, there can also be risks if the interior plumbing has old lead piping that carries water to the faucet.

Prior to 1987, plumbers used something called "lead solder" to connect external piping to a house's internal piping. And local water departments say that is a risk factor as a possible lead source.

In 1986, Congress enacted the "lead ban," which stated that anyone who installs or repairs plumbing that carries drinking water must use lead-free materials. As a result, homes built in or after 1986 are far less likely to have lead solder materials. But these regulations don't apply to older houses, which is why there is still lead piping around today.

5) Water is not the main cause of lead poisoning overall

Lead in the drinking water is one of the contributors to the total lead exposure, but the main cause of chronic lead poisoning is lead-based paint in old homes. When the paint deteriorates, it breaks in little chips that mix with household dust and can be inhaled or ingested.

There are other causes — like painted toys, some cosmetics, and pottery. The EPA has a list of lead exposure causes here.

No matter how small, there is no safe level of lead exposure. The key to prevent harm is to prevent exposure in the first place.