It will be a little harder for some people to breathe today because of heat combined with high levels of air pollution, says a statement from Environment Canada.
The agency has issued a special air quality statement for southwestern Ontario, including Hamilton.
It says to expect possible high levels of air pollution, which will cause "deteriorating air quality."
"Hot and sunny conditions are expected to cause increasing ground-level ozone concentrations in the area," it says.
This will continue this afternoon, and possibly into Monday afternoon.
That means people could experience increased coughing, throat irritation, headaches or shortness of breath. Children, seniors and those with cardiovascular and lung disease are particularly at risk.
Anyone with those symptoms should hold off on strenuous outdoor activities.
The weather today will be 28 C with a Humidex of 33. This afternoon brings a 30 per cent chance of showers and a risk of a thunderstorm.
Sunday night will be partly cloudy with a 30 per cent chance of showers and risk of a thunderstorm early in the evening. Fog patches will develop overnight. The low will be 17 C.
Monday will be sunny, with sun and cloud in the afternoon. Fog patches will dissipate early in the morning. The high will be 30 C and the Humidex 36.