Eighteen people have died in six days from a heat wave in and around Montreal in Canada, while conditions are forecast to become “even more uncomfortable” on Thursday. The toll in Montreal of heat-related deaths was 12, while six people have died in Quebec’s Eastern Townships, CBC reported.

The heat wave started on June 29 and is said to be the worst to hit Quebec province in decades. Montreal is the largest city of the province.

In its Thursday dawn update, the government said the “warm and humid airmass over Southern Quebec will persist” for the day. However, “the passage of a vigorous cold front Thursday night will put an end to this prolonged period of extreme heat”, the update said.

Officials have urged residents to check on their neighbours and loved ones, especially those without air conditioning. “The record temperatures are expected to continue in central and eastern Canada, so make sure you know how to protect yourself and your family,” Prime Minister Justin Trudeau said on Twitter.

Most of those who died lived alone and none had access to air conditioning, David Kaiser, a physician at Montreal’s public health department, told Reuters. The city authorities have raised the response level to “intervention” from “alert” earlier.

The maximum temperature in Montreal on Wednesday was 34 degrees Celsius, and the city is likely to cool to 24 degrees on Friday. However, the humidity index is likely to rise to 43 on Thursday. Canadian meteorologists use the index, or “humidex”, to combine the heat and humidity measurements to estimate what the temperature actually feels like. A value between 40 and 45 indicates “great discomfort”, and anything above that is dangerous.

Montreal Mayor Valérie Plante said “we are doing everything we can”. The city has opened swimming pools and air-conditioned areas to the public, and is distributing water to those in need and has first responders checking in on vulnerable citizens, CBC reported.