What is Humidex and How Can You Measure It?

Humidex is a weather metric used to describe how hot it feels when humidity is factored into the air temperature. It was developed in Canada during the 1960s and refined around 1979 by J. M. Masterton and F. A. Richardson. The purpose is simple: when humid and hot conditions combine, our ability to cool down through sweating is reduced, making it feel significantly warmer than the air temperature alone suggests. Humidex gives a single number that reflects that feeling.

This post will explain how Humidex is calculated, why it matters for health and safety, how to interpret its values, how to measure it properly, and what industries or situations benefit from using it.

How Humidex Differs from Other Heat Measures

Humidex is often compared with the heat index, which is used in the United States, but there are key differences.

Heat index combines air temperature and relative humidity to estimate how hot the air feels under shaded conditions. However, it does not use the dew point, and it ignores radiant heat, which is the heat you feel directly from sunlight or hot surfaces.

Humidex uses the dew point rather than relative humidity. The dew point indicates the amount of water vapor in the air and the temperature at which it would saturate. When humidity is high, sweat does not evaporate well, so the body is less able to cool itself.

That makes Humidex particularly meaningful in hot, humid areas and for work or activity settings where heat exposure is continuous. It gives a more realistic sense of discomfort and risk than air temperature alone.

How Humidex Is Calculated

Numerous formulas exist, but the common version used by Environment Canada is:

Humidex = T_air + 0.5555 × (e - 10)

Here, T_air is air temperature in degrees Celsius, and e is the vapour pressure in millibars, which is calculated using the dew point. The formula for e is:

e = 6.11 × exp [5417.7530 × ((1 / 273.15) - (1 / (dew_point + 273.15)))]

In that calculation, the dew point is converted into Kelvin by adding 273.15. The constant 5417.7530 comes from physical properties of water vapor, latent heat of evaporation, and universal gas constants. The value 0.5555 is approximately the conversion factor from the original formula, which used Fahrenheit, and has been adjusted for Celsius.

Because the dew point must be measured, calculating Humidex depends on having accurate temperature and moisture sensors.

How to Interpret Humidex Values for Comfort and Risk

Humidex values are grouped into ranges to indicate comfort or risk. These ranges help determine safe activity levels, when to seek shade, when to rest, and when to avoid unnecessary exposure. The common ranges in Celsius are:

• 20 to 29: little to no discomfort
• 30 to 39: some discomfort
• 40 to 45: great discomfort; avoid exertion
• Above 45: dangerous; heat stroke possible

These groupings mean that at lower values, people will generally feel warm but tolerable. In the 30s, sweating, fatigue, and irritability become common. In the 40s, physical exertion could lead to serious risk, especially for those not acclimatized. When it exceeds 45, the risk of heat illness becomes serious, and preventive measures are essential.

How to Measure Humidex in Practical Settings

To use Humidex usefully, measurement must be accurate and relevant to the location and activity. Here is how to do that well:

Measure air temperature with a reliable thermometer rated for outdoor or indoor use, depending on the environment.

Measure dew point. The dew point can be found via a sensor or calculated from the relative humidity and temperature.

Ensure measurements are taken at or near the height where people will be working or active.

Take readings frequently, especially during hours when temperature or humidity may change. Hourly when above certain thresholds.

Adjust for clothing and equipment. Heavier clothing, protective gear, or non-breathable fabrics increase heat burden and add to the Humidex effect.

Adjust for radiant heat or direct sunlight exposure by applying a correction factor. For many workplace guidelines, this means adding a few degrees when direct sun is present.

Use tools such as digital hygrometers or wet and dry bulb thermometers to get humidity and temperature.

Why Humidex Matters for Health, Safety, and Workplaces

Humidex is not only a comfort metric; it is also a safety metric. It has a real impact on health, safety, and productivity.

When Humidex values are high, the risk of heat exhaustion, dehydration, heat cramps, and even heat stroke increases.

Workplaces with manual labor outdoors, warehouses with poor ventilation, kitchens, event venues, and athletic training sessions all benefit from Humidex measurement.

Knowing Humidex enables organizations to plan rest breaks, schedule heavy work for cooler times of day, increase access to hydration, avoid overexertion, and protect vulnerable individuals, such as older adults or those with health conditions.

Regulatory bodies in regions that observe Humidex or similar indices often recommend guidelines or response levels triggered by Humidex readings to protect workers.

Industries and Scenarios Where Humidex Is Especially Useful

Some industries and situations gain greater benefit from monitoring Humidex:

• Indoor workplaces without air conditioning but with high humidity, such as greenhouses, factories, laundries, or kitchens.
• Outdoor work in construction, landscaping, utilities, and roadwork.
• Sports and fitness training outdoors, especially in full sun or high humidity.
• Event planning and venue management for outdoor festivals or concerts.
• Emergency response teams responding during heatwaves.
• Shipping, logistics, and cold supply chain workers who might face heat plus humidity inside containers or loading docks.

Comparing Humidex and Other Heat Indicators

Humidex is one useful indicator among several. Understanding when to use each helps improve decision-making.

Humidex works best when you need a simple sense of how uncomfortable or risky weather feels due to the combination of heat and moisture.

For more detailed safety planning in extreme heat or physical exertion environments, consider using the Wet Bulb Globe Temperature (WBGT), which accounts for the effects of radiant heat, wind, and work intensity.

Also consider the heat index, which is simple and well-known, though less precise under some conditions.

Using Humidex in conjunction with data such as humidity, air movement, and personal factors (including clothing and acclimatization) provides a comprehensive risk assessment.

What You Should Know

Humidex provides a calculation that combines air temperature and dew point to better express how hot it feels when the air is saturated with moisture.

It is more accurate than air temperature alone and often more meaningful than heat index in humid environments.

If you measure it correctly and respond to elevated values by adjusting work schedules, rest breaks, hydration, and clothing, it can significantly reduce the risk of heat illness and improve comfort.

Knowing Humidex can help workers, coaches, safety managers, and event organizers make better decisions in hot, humid conditions.