“It’s not the heat, it’s the humidity” …Actually, it’s both!

Ever notice how some hot days feel way more intense than others, even though your thermometer shows the exact same temperature? That’s humidity at play. When it’s hot, your body tries to stay cool by sweating. As sweat evaporates, it takes heat with it, cooling you down. But when the air is already full of moisture – a.k.a high humidity – sweat doesn’t evaporate as easily. So instead of cooling down, your body just stays hot and sticky (1).

This is where the humidex or heat index comes in. These numbers tell us how hot it feels to our body when we combine temperature and humidity (1,2). For example:

35°C with high humidity can feel more like 45°C.

Similarly, if the humidity is low (~15%), 37.7°C can feel more like 35.5°C (1,3). 

When the humidex is above 39, heat can become very uncomfortable and it’s best to avoid physical activity. Above 45? Heat stroke becomes a serious risk (2).

That’s right, humidity doesn’t just make things uncomfortable, it can be dangerous. If it’s high enough, it can make your body’s cooling system shut down. When relative humidity hits 90%, sweat stops evaporating entirely (2). That means your body can’t cool itself at all, and your internal temperature rises, sometimes to the point of illness (check-out our post on heat-related illness – 4). 

This is especially risky for people with asthma, chronic obstructive pulmonary disease, or other conditions. High heat and humidity can make air more stagnant, trapping pollutants that can worsen breathing problems (5).