Toronto is under its first heat warning of the year.

City dwellers rushed to turn up the air conditioning on Sunday as temperatures climbed past a forecasted high of 32 C with the humidex making it feel like 40.

Environment Canada issued the warning for Toronto and much of southern Ontario early Sunday. A special air quality statement was also in effect for the city. It warned of possibly high levels of air pollution and "deteriorating" air quality as the city heated up.

The sunny and clear sky conditions are also expected to cause increasing ground-level ozone concentrations in southern Ontario, the federal weather agency added.

That means added risks to young children, pregnant women, older adults and those working outdoors.

Toronto has issued a heat warning for Sunday and opened seven cooling centres. (Muriel Draaisma/CBC)

The city has opened seven cooling centres as a result of sweltering temperatures. These air-conditioned facilities allow people to rest and receive a cool drink and light snack.

Meanwhile, the outdoor swimming season began just in time to make the blast of warm weather more bearable. Some outdoor pools opened early on Saturday — 10 in total. The remaining pools will open as scheduled, one week later. All dates are weather permitting, the city says.

Heat wave continues Monday

The heat warning is expected to stick around into Monday with a high of 31 C, the federal weather agency says. But that will feel closer to 40 with the humidex.

"It won't give us a relief from the heat," said Haizhen Sun, a severe weather meteorologist for Environment Canada.

But a break from the high temperatures might come Monday night when a cold front is expected to sweep across southern Ontario, she added.

"Tuesday will be much more pleasant," said Sun.

Summer officially begins on June 21, and this year is expected to be a scorcher.