What you need to know about boil water advisories

Thousands of people living on Montreal's South Shore have been ordered to boil their water before consuming it. Authorities say it's because tests found elevated levels of fecal coliforms.

What does that mean? And is it still safe to use tap water for anything?

CBC Montreal spoke to Sarah Dorner, who holds the Canada Research Chair in source water protection and is an associate professor of civil, geological and mining engineering at Montreal's École Polytechnique.

What are fecal coliforms?

Fecal coliforms are bacteria that generally come from the intestines of warm-blooded animals.

They're easy to test for in water, and if they're found it could mean that the water has been contaminated by feces.

If a test comes back positive for fecal coliforms, there may be other bacteria and pathogens in the water which could make people sick.

Are they dangerous on their own?

Yes and no.

Fecal coliforms are a group of bacteria. E. coli is part of the group that make up fecal coliforms and pose a risk when consumed.

In 2000, an E. coli contamination in the Walkerton, Ont.'s water supply killed seven people and made 2,300 people sick.

If a municipality has a sample that tests positive for fecal coliforms, it will put in place a boil water advisory as a precaution.

For how long should you boil your water?

A one-minute rolling boil is the widely accepted standard and is recommended by Health Canada and The Centers for Disease Control and Prevention in the United States.

A minute will even kill the hardier micro-organisms that might be present when fecal coliforms are found. Anything longer than a minute will just be wasting the energy used to bring the water to a boil.

Freezing might not kill all micro-organisims. That's why it's recommended to throw out any ice cubes which may have been made with contaminated water.

Can you cook with contaminated tap water?

If you're using the water to cook something that will come to a boil for over a minute, like pasta or soup, there should be no concerns. However, if you're just rinsing produce or preparing something like baby formula you should use water that has been boiled and set aside – or bottled water.

What other precautions should you take?

It is still OK to take a shower or bath: Skin contact with tap water is not a concern. However, children should be supervised in case they should try to drink the water being used.

You should not use tap water to brush or rinse your teeth since it can pass on whatever micro-organisms that may be present. You should use water that has been boiled and set aside or bottled.

Washing your dishes in tap water is safe. The pathogens found in contaminated water need a certain environment. Nothing should survive after being washed with hot water and dried off. The same applies to washing your laundry.

What about pets?

It's a matter of personal choice. Animals have a different immune system and are exposed to all sorts of micro-organisms on a daily basis. For example, they might drink from a puddle while out on a walk.

There's no harm in providing boiled or bottled water to a pet, but it's probably not necessary.