The Middlesex-London Health Unit has issued a one-day heat alert for Friday as high temperature and humidity are expected to make conditions hot and sticky.
This is the second heat alert of the year, coming just one week after the last.
Environment Canada is calling for a high of 31C on Friday, with the humidex pushing that up to 41C.
Randy Walker, public health inspector with the MLHU, said in a statement, "The 25th edition of Sunfest is opening this weekend in London, so we know there will be a lot of people enjoying the sounds, sights and tastes of the world at Victoria Park. With this forecast, we also know they’ll need to find ways to beat the heat."
Whether you're at the festival, camping or just enjoying the outdoors, Walker recommends keeping a water bottle close by, using sunblock or covering exposed skin with light clothing and taking breaks from the heat in the shade or an air-conditioned space.
The health unit also recommends avoiding alcoholic beverages, coffee and cola, intense physical activity, heavy meals.
They are also reminding people that children and pets should never be left in parked cars or sleeping outside in direct sunlight.
More information on heat-related illness is available on the MLHU website.