Heads up if you're thinking about planning a waterfront picnic in Toronto's west end sometime next month: Maybe don't.

Toronto Public Health announced earlier this week that a potentially toxic type of blue-green algae has been detected in Humber Bay and around Mimico creek.

Called cyanobacteria, the plant-like substance forms naturally in ponds, rivers and lakes from time to time. You can easily see blooms of it in the water, but shouldn't get close.

"Some species of blue-green algae have the potential to produce toxins that are harmful to the health of humans and animals," reads a press release from the City of Toronto, noting that exposure can cause headaches, fever, diarrhoea, abdominal pain, nausea, vomiting, skin rashes and mucous membrane irritation in humans.

This is not blue green algae, this is more like toxic sludge. Killing all the wildlife & marinelife...We need your help to help get us heard❤️🙏🏻 pic.twitter.com/gf6AyyJoQR

— Heide Horvath (@HeideHorvath) July 16, 2018

"Adverse health effects are mainly caused by drinking and coming into contact with water that is contaminated with blue-green algae toxins," the public health agency says.

Fortunately, it's easy to avoid the stuff: just don't go into or drink the lake water. What can't be avoided is the smell, which can apparently get pretty gross as the algae blooms age.

TPH said on Wednesday that, while fresh blue-green algae smells like fresh-cut grass, "they may start to smell like rotting garbage" later in the summer, particularly when it's hot. 

NEWS RELEASE: Toronto Public Health issues warning concerning blue-green algae blooms in Humber Bay East. More info: https://t.co/ppILSlXIjw

— Toronto PublicHealth (@TOPublicHealth) July 17, 2018

People in other areas of North America affected by blue-green algae have called the smell "foul" and "offputting," likening it to sewage or an overflowing septic tank.

"Some of the really bad smells may come from decomposing critters, like mussels that are carried ashore tangled in the mass of vegetation," reads an explainer from Wisconson Natural Resources  Magazine. 

"These decomposing little animals can smell even worse than the algae, just as we've all observed when cleaning out a refrigerator – the smells of rotting meats and other proteins are usually stronger than the odors from decomposing fruits and vegetables."

Gross.