You may want to think twice about that midnight run to get a drink of water from the sink.

The Environmental Working Group, a non-profit research group, recently examined data on tap water in the Bay Area and throughout the U.S. Their findings, published in an online database, found some unsettling information about the tap water most of us drink everyday.

A total of 13 contaminants were found to be over the health guidelines in at least one of the water districts serving major Bay Area markets. Many of these contaminants, in high enough concentrations, can cause cancer and difficulties during pregnancy.

"The vast majority of the nation's drinking water supplies get a passing grade from federal and state regulatory agencies," the EWG writes. "However, many of the 250-plus contaminants detected through water sampling and testing are at levels that are perfectly legal under the Safe Drinking Water Act or state regulations, but well above levels authoritative scientific studies have found to pose health risks."

The Environmental Working Group selects the health guideline from those established by federal or state public health authorities. This is often different than the legal limit of a substance in water.

"Just because your tap water gets a passing grade from the government doesn't always mean it's safe," said EWG President Ken Cook in a statement. "It's time to stop basing environmental regulations on political or economic compromises, and instead listen to what scientists say about the long-term effects of toxic chemicals and empower Americans to protect themselves from pollutants even as they demand the protective action they deserve from government."

In the Bay Area EWG found arsenic, bromodichloromethane , bromate, bromoform, chlorate, chloroform, chromium (hexavalent), dichloroacetic ccid, hormones, manganese, radiological contaminants, trichloroacetic acid and total trihalomethanes above the guidelines in different areas.

The level of contamination varied. Hexavalent chromium levels were only 0.02 ppb higher than the guideline in Hayward, while in Daly City they were 7.68 ppb higher. This put Daly City at more than nine times the national average level of hexavalent chromium.

In order to build the database, researchers at EWG gathered data on almost 50,000 public water systems throughout the U.S.

"Americans deserve the fullest picture possible of what's in their tap water," said Cook in a prepared statement. "But they won't get that information from the government or, in many cases, from their utilities. The only place they'll find that is EWG's drinking water report."

While chloroform and arsenic might be familiar, in a time when various studies are saying everything from cell phones to pickles cause cancer, you may be wondering what exactly these chemicals do, and how they got in your water.

Scroll down for that information, and click through the slideshow to see what was found where in the Bay Area, according to the Environmental Working Group.

Why are they bad, and where do they come from?

Arsenic may cause cancer and is responsible for thousands of cases each year in the U.S. It can be a byproduct of agriculture and industry, but also occurs naturally.

Bromodichloromethane may cause cancer, and could cause problems during pregnancy. It is a result of disinfectants used to treat drinking water.

Bromate has caused DNA damage and cancer in laboratory animals. It is a byproduct of when water that already contains bromide is treated with ozonation or sodium hypochlorite.

Bromoform may cause cancer and could cause problems during pregnancy. It forms during when water is disinfected.

Chlorate is harmful to the thyroid, which makes it especially dangerous to children and pregnant women. Although one contaminant can come from a variety of sources, chlorate typically forms as a byproduct of disinfection.

Chloroform causes cancer, and may cause problems during pregnancy. It is a result of disinfectants used to treat drinking water.

Chromium (Hexavalent) may cause cancer. Although one contaminant can come from a variety of sources, Hexavalent Chromium typically is the result of industry pollution, or, in some areas, mineral deposits and groundwater.

Dibromochloromethane may cause cancer, and problems during pregnancy. It is a result of disinfectants used to treat drinking water.

Dichloroacetic Acid may cause cancer, and problems during pregnancy. It is a result of disinfectants used to treat drinking water.

Hormone containments come from animal or human wastewater flowing into water sources.

Manganese is harmful to the brain, and may impair children's attention, memory and intellectual capacity. It is a naturally occurring element found in food and drinking water, and excessive exposure is necessary for these effects.

Radiological contaminants, increase the risk of cancer and may cause fetal development issues. Typically, these containments are the result of mining, or certain minerals.

Trichloroacetic Acid may cause cancer, and may cause problems during pregnancy. It is a result of disinfectants used to treat drinking water.

Total Trihalomethanes are a group including chloroform, bromodichloromethane, dibromochloromethane and bromoform, which can cause cancer and may cause problems during pregnancy. They form during when water is disinfected.

Fernando Ramirez contributed to this report.