Residents are being advised to cool off at pools and splash pads. ((Canadian Press) )

Ottawa's medical officer of health has issued a heat warning that will stay in effect until further notice.

A heat warning is automatically declared in the city when Environment Canada forecasts a humidex of 40 or more for at least two consecutive days.

Extreme heat can cause dehydration, heat exhaustion, heat stroke — even death.

"Heat is hardest on the elderly and chronically ill. If they have no protection from the heat, such as access to air conditioning and plenty of fluids, they can develop serious health effects and even die," said Dr. Isra Levy, in a statement issued Monday afternoon.

The heat warning is an upgrade of the heat alert that Ottawa Public health issued earlier in the day when temperatures hit 30 plus degrees for the second day in a row.

The city is advising residents to cool down at pools and splash pads, community centres and libraries.

The hot weather is expected to continue until Thursday.