Calculating Heat Stress Time-Weighted Averages

During a heat dome like we’re in right now, the outdoor (or indoor) Humidex can exceed 45.  Looking at the OHCOW Humidex-Based Heat Response Plan, it appears that when you exceed a Humidex of 45 it’s “game over”.  Medical monitoring above 45 involves measuring a worker’s body temperature and/or heart rate and comparing that to physiological standards (<38.5°C and heart less than [180 bpm (beats per minute) – age] for healthy individuals with normal cardiac response).  Anyone who collects such information should be under the direct supervision of a recognized medical professional who will take responsibility for how the trained person will collect the information, how they store it confidentially, and what actions to take in response.  This is often well beyond what most workplaces can provide, so what can they do?

First of all, the evaluation time period for assessing heat stress is averaged over 1 hour.  So, you need to log the amount of time a worker spends in different places over the hour.  Let’s say it’s really hot outside (like yesterday in Brantford at 4 pm), temperature = 33.3°C; relative humidity (RH) = 56% and it’s sunny; therefore, the adjusted Humidex = 44 +4 = 48 (the extra 4 Humidex units adjusts the Humidex to take into account being in direct sunlight).

Let’s take the example of a lifeguard at an outdoor pool.  Looking at the table below, you might conclude that only medically monitored workers can continue to work.  However, let’s suppose the lifeguard spends 15 minutes of each hour inside an air-conditioned space at 25°C and 65% RH (Humidex =31).  Then, the time weighted average heat exposure for the whole hour would be 45 min/60 min at a Humidex of 48 and 15 min/60 min at a Humidex of 31, which works out to be ((45/60 min  x  48)  +  (15/60 min  x 31) =) a sun-adjusted, time-weighted average Humidex of 44.  Looking at the chart below, at a Humidex of 44, the plan prescribes 45 minutes of rest time whereas the lifeguard was only getting 15 minutes each hour.  So, 15 minutes rest inside is insufficient to safeguard the lifeguard from overheating.

So, let’s increase the relief time to a half hour inside the air-conditioned office and redo the calculations.  The sun-adjusted, time-weighted average Humidex would then be ((30/60 min  x  48)  +  (30/60 min  x 31) =) 40 which corresponds to a 30 minutes/hr relief and 30 min/hr work.  So, we are now meeting the Humidex plan recommendations.

Time Weighted Average Humidex Calculations: video by John Oudyk

Download PDF Slides:

* “adjusted” means adjusted for additional clothing and radiant heat (add 4 to the Humidex value to account for direct sunlight)

** at Humidex above 45, heat stress to be managed as per the ACGIH TLV^®

For those who would like a spreadsheet calculator to help with this computation of a time-weighted average, please contact John Oudyk at joudyk@ohcow.on.ca to request a copy.