The Hamilton, Halton and Niagara regions are being hit with extreme heat warnings that could have the humidex reaching the 40s in the days to come.

Environment Canada has warned that the heat could last into Thursday, with the eastern cities in southern Ontario expected to hit 40 degrees Celsius before the humidex.

The heat is expected to stretch throughout Southern Ontario reaching from Windsor, to Cornwall and as far north as Fort Hope.

Officials are also warning that the heat could impact air quality amidst some of the worst air quality levels being experienced in the regions in the past week.

WATCH MORE: Hamilton, Halton, Niagara regions among worst for air quality in Ontario

The City of Hamilton’s Medical Officer of Health officially instated a heat warning in the city as of Tuesday.

Along with the warning, the City is reminding residents to stay cool with public cooling centres opening in libraries and recreation centres and visits to public pools, which are free during the heat warning.

Hamilton city officials have encouraged landlords to ensure cool spaces for tenants, meaning providing properly air conditioned units, or air conditioned common spaces.

Parents, guardians and pet owners are additionally reminded that leaving children and pets in hot vehicles during this time can be deadly within minutes.

Residents in these regions are urged to keep track of any signs of heat exhaustion including heavy sweating, weakness, cold, pale and clammy skin, weak pulse, fainting and vomiting. Anyone experiencing any of these symptoms are asked to call 911 immediately.

READ MORE: Road closure coming Tuesday on Burlington’s Lakeshore Rd.