Montreal Mayor Gérald Tremblay closed more than half of the city's 73 outdoor pools after a study showed they were heavily contaminated with dangerous bacteria. (CBC)

On a hot summer day, taking a dip in the pool is a refreshing escape from the heat.

But "fresh" might not be the right word if your swimming companions include bacteria that can cause vomiting, diarrhea and skin disease.

On Aug. 22, Quebec Environment Minister Claude Béchard announced all public pools in the province would be tested for chlorine and acid levels every three hours. Tests for bacteria levels would be done twice a month.

The new rules will come into effect in the coming months. Earlier in the week, lab tests showed outdoor swimming pools in Montreal and Laval, north of Montreal, were heavily contaminated with bacteria, viruses and fecal matter.

Test results must be submitted to the provincial government, Béchard said. Municipalities are responsible for pool safety in Quebec, but the province has the power to close facilities.

Elsewhere in Canada, documents obtained by CBC News showed that Winnipeg's environmental health inspectors shut down 13 pools and whirlpools in 2004 because of bacteria contamination, chlorine imbalance or unsafe conditions.

And in Nova Scotia in 2003, a former public health officer sampled water in 19 public swimming pools and found unacceptably high levels of bacteria. The results failed to meet provincial guidelines.

It's a scary prospect that the local swimming pool could be a veritable microbial soup, bubbling with germs and parasites.

Where do the organisms come from?

The risk of illness and infection from swimming pools derives mostly from fecal contamination of the water. Vomit, mucus, saliva and skin are also sources of dangerous germs in the pool.

Sometimes these substances are released into the water by bathers, or they can come directly from birds and rodents. Viral outbreaks can also come from inadequate pool sanitation. Filtering the pool water and adding chlorine, salt and other disinfectants will usually control harmful germs in the water.

Gary Sanger, aquatics supervisor for the City of Toronto, says the city has a detailed process to keep the pools contaminant free. Pool officials test the water a half hour before a pool is open to the public, and every two hours thereafter. If there are any foreign substances such as vomit in the water, Sanger says, the pool is closed for cleaning and chlorination.

During the summer it becomes more difficult to control the bacteria, says Dr. John Carsley of the Montreal Public Health Department.

"It's a challenge to keep the chlorine at the right level, from what I understand," Carsley said. "Especially, the hotter it gets, the quicker the chlorine gets used up."

The more swimmers splashing in the water, the harder it is to maintain the appropriate levels of chlorine to keep the germs at bay.

"The more people who get in the pool and the more demand in terms of microbial burden there is," he says.

The hotter the water the temperature, the better the habitat for bacteria. Hot tubs and natural spas with mineral water are an ideal breeding ground for germs such as legionella.

What kind of illnesses can I catch from swimming in a contaminated pool?

E.coli can lead to bloody diarrhea, as well as vomiting and fever in more severe cases. Elderly people and infants can develop haemolytic uraemic syndrome (HUS), which can lead to kidney failure. Shigella bacteria can cause diarrhea, fever and nausea.

Illnesses can also stem from parasites found in fecal matter in the water. The most common are Giardia and Cryptosporidium, or C. difficile, according to the World Health Organization. They are both highly contagious because infected individuals shed the sickness-causing germs in large amounts. Giardiasis is characterized by diarrhea, cramps, foul-smelling stools, loss of appetite, fatigue and vomiting. Symptoms of cryptosporidiosis include diarrhea, vomiting, fever and abdominal cramps.

It is difficult to directly link outbreaks of illness with pool water because the evidence is usually circumstantial, according to the WHO, such as a pattern between swimmers' symptoms and where they last swam. It's difficult to isolate these microbes from the water itself.

Are there pool sanitation regulations to control the bacteria levels in public pools?

There aren't nation-wide regulations for pool sanitation in Canada, but each province has its own set. They typically include testing the bacteria levels in the pool before opening it to the public, testing it regularly while people are swimming, and cleaning and disinfecting the pool regularly.

The World Health Organization also has a set of recommended guidelines for safe swimming.

What can I do to have a healthy and safe swim?

Both pool and swimmers have their part to play, Sanger says. Pool officials can control the contamination of pools by encouraging swimmers to shower before diving in, he says.

Sanger encourages parents to bathe their kids before bringing them into the pool. And he advises young children use disposable swim trunks designed to keep unwanted substances under wraps.

The World Health Organization recommends keeping small children in pools small enough to drain, in the event of accidental defecation.

As well, people with gastroenteritis — inflammation of the gastrointestinal tract or the pathway responsible for digestion including the mouth, esophagus, stomach and intestines, also known as the "stomach flu" — should not go to public or semi-public pools until at least a week after their illness.

Is bacteria in public pools common problem? Should I be worried?

No, says Sanger, who adds it's a fairly infrequent problem, and Toronto is "vigilant" in following the provincial guidelines regarding pool sanitation. He adds there is no reason to believe that Toronto's pools are dangerous.

Carsley says that Montreal's department of public health hasn't had any reports of outbreaks of infectious disease stemming from the city's pools.

"From time to time, we have the opposite," he says. "Particularly with indoor pools we might have, once or twice a year, a problem with too much chlorine. That causes irritation and people feeling bad all at once. So, it's somewhat easier to detect than, say, a few cases of diarrhea from which the links may not be made. But I think with significant outbreaks related to pools, we would hear about it one way or another: through physicians, through day care, through day camps, through the pool itself."

Swimmers need not avoid the local watering hole, Sanger says, and educating people about keeping bacteria out of the pool — by bathing before taking a dip, and staying out if you're sick — is a better approach.