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Occupational Health Clinics for Ontario Workers Inc. Humidex Based Heat Response Plan
What is it? Ø the Humidex plan is a simplified way of protecting workers from heat stress which is based on the ACGIH Heat Stress TLV (used by the Ontario Ministry of Labour) Ø wet bulb globe temperatures (WBGT) were translated into Humidex based on the “moderate” work load category & assuming workers are unacclimatized (see rationale at the end of the document)
Note: in the translation process some simplifications and assumptions have been made, therefore, the plan may not be applicable in all circumstances and/or workplaces (follow steps #1-5 to ensure the Humidex plan is appropriate for your workplace). Humidex Heat Stress Response Plan
Humidex calculator: http://www.hchsa.on.ca/new/OHCOWHumidex/calc.html Fans: Fans provide air movement which can increase the rate at which sweat evaporates (thus cooling the body). However, when relative humidity levels rise above 70%, very little evaporation occurs and increasing air movement has little benefit. If the air is the same temperature as the skin (36ºC) or higher, moving air may actually heat up the body especially if the humidity is high. Vulnerability to Heat Stress: There are many permanent or temporary conditions (e.g. age, heart or lung conditions, dehydration, fatigue, some medications, etc.) that can make a person more vulnerable to heat strain. Despite their condition, they may be able to cope given adequate knowledge of the signs and symptoms of heat stress and if given the latitude to make the appropriate adjustments to their workplace or work routine. It is more often the young, fit workers who may think they are invincible who succumb to heat strain. Some workers may need medical advice about what accommodations would be right for them.Humidex Based Heat Response Plan Step #1: Clothing Ø evaporating sweat is the primary way the body gets rid of excess heat build-up, therefore, the best clothing is the kind that makes it easiest for sweat to evaporate. Ø the Humidex plan assumes workers are wearing regular summer clothes (light shirt & pants, underwear and socks and shoes). Ø for workers who wear cotton overalls on top of summer clothes one should add 5ºC Humidex (roughly equal to 3.5ºC WBGT) to the workplace Humidex measurement. Ø for different clothing configurations, estimate correction factor by comparing them with cotton overalls (e.g. gloves, hard hat, apron, protective sleeves might be equivalent to a little less than half the evaporation resistance as overalls so add 1ºC or 2ºC Humidex).
Step #2: Training Ø the Humidex plan by itself cannot guarantee that workers will not be affected by heat stress. It is absolutely essential that workers learn to recognize the early signs and symptoms of heat stress and know what to do to prevent them! Ø if at all possible, workers need to be able to alter their pace of work, rest breaks, and fluid intake in response to early symptoms (240 mL every 20 minutes). Ø the ideal heat stress response plan would let workers regulate their own pace by "listening to their body" without need for measurements.
Step #3: Select a Measurement Location Ø split the workplace into heat stress zones and put a thermal hygrometer in each zone. Ø identify a representative location within the zone where measurements can be taken (if you want to base your actions on a single reading, select the highest heat stress zone).
Note: the Humidex Heat Stress Response Plan is based on workplace measurements not weather station/media reports (temperatures inside buildings do not necessarily correspond with outside temperatures)
Step #4: Measure Workplace Humidex Ø a thermal hygrometer (usually $20-$60 at hardware or office supply stores) is a simple way to measure the temperature and relative humidity in your workplace Ø once you have the temperature and humidity, use the table above to determine the corresponding Humidex value and the appropriate heat stress prevention response Ø measurements should be recorded at least hourly if the Humidex is above 30ºC NEVER IGNORE ANYONE'S SYMPTOMS NO MATTER WHAT THE HUMIDEX! Step #5: Adjusting for Radiant Heat Ø for outdoor work in direct sunlight between the hours of 10 am and 4 pm, add 2-3ºC (pro-rate according to percentage cloud cover) to your Humidex measurement for indoor radiant heat exposures, use common sense to judge whether the exposure of concern involves more or less radiant heat than direct sunlight and adjust the 2-3ºC correction factor appropriately
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