Millions of people are under a heat warning in Ontario, Quebec and New Brunswick, with Environment and Climate Change Canada warning that temperatures could reach as high as 35 C. The weather service is warning of “dangerously hot and humid conditions” in parts of Ontario and Quebec in particular. …Humidex values, which combine the air temperature with humidity to calculate what heat feels like to the average person, could reach 45 C in parts of Quebec and Ontario. …Dave Phillips, with Environment and Climate Change Canada, described the system as a massive heat dome, which is a high-pressure system that works to trap high temperatures near the Earth’s surface. The heat rises and then gets pushed back down, like a convection oven, he said. …Extreme heat is a major health risk, particularly for older adults, infants and young children, and people with disabilities or mobility issues.