The presence of coliforms in drinking water indicates the possible presence of organisms that can cause diarrhea, cramps, nausea and headaches. In 2015 alone, community water systems serving 17.7 million Americans (that’s 1 in 20) confirmed more than 10,000 violations of the rules protecting your tap water from these bacteria. Do you live in one of those communities? Chances are you don’t know.

Exposure to contaminants like chlorine and sodium hypochlorite, the active ingredient in household bleach, can lead to cancer and may be linked to reproductive harms like miscarriages and birth defects. In 2015, there were more than 11,000 violations at U.S. community water systems that serve 25.1 million people — that’s 1 in 14 Americans. Are you pregnant, or do you think you might be pregnant? Ask your doctor what to do to protect your developing child.

Nitrate exposure can change how oxygen is carried in the bloodstream, leading in some cases to potentially fatal “blue baby syndrome” in infants. It can also increase the risk of spina bifida and other birth defects, thyroid disease and colon cancer. In 2015, there were more than 1,500 violations at community water systems serving 3.8 million people.

Exposure to lead is particularly toxic to children and can cause serious, irreversible damage to their developing brains and nervous systems. Lead exposure can also cause miscarriages and stillbirths, fertility problems and cognitive dysfunction. 18.3 million Americans got their tap water from utilities that racked up more than 8,000 violations of the federal government’s Lead and Copper Rule. Those utilities pump water to schools and assisted living facilities.

Exposure to radioactive materials can lead to cancers and compromised kidney function. In 2015, there were more than 2,200 violations in community water systems serving 1.4 million people.

Arsenic is a carcinogen that can contribute to cancers, developmental harm, and pulmonary and cardiovascular diseases. In 2015, there were more than 1,500 reported violations at community water systems serving 1.8 million of us. Arsenic is also a powerful rat poison. And as with other contaminants, it’s hard to reliably measure at home. That’s because your water utility flushes their pipes and pumps water out at different rates on different days, so the level on Monday might not match what’s coming out of your tap on Tuesday.

Exposure from pesticides and other man-made organic chemicals can lead to cancer, developmental harm, central nervous system and reproductive diseases, endocrine issues, and liver and kidney problems. In 2015 there were more than 6,800 violations recorded at water systems that serve 2.6 million Americans. The most common one is called atrazine. And it can harm your reproductive system, scrambling how much estrogen and testosterone reaches our bloodstreams. Some tests have shown that it can change the sex of some animals.

A biofilm forms when bacteria, fungi, amoebas and other microorganisms cling together into a slimy goop that coats the inside walls of plumbing pipes. It’s a haven for organisms we don’t want in our water supply, like the waterborne pathogen that causes Legionnaires’ disease. The glue-like paste can also protect bacteria from disinfectants that water utilities routinely use. You can give yourself a quick introduction to biofilms by running your tongue over your teeth if you haven’t brushed for a few days.