Heat stress monitors

HOT ENOUGH FOR YA?

By Hayley Chandler

A scorcher. A real sizzler. Whatever we’ve called it in the past, extreme heat on the job site is finally getting the name it deserves: health hazard.

While a hot day at the beach may be heavenly, a hot day on a construction site or in a foundry can be hell. It’s not only unpleasant, it’s potentially dangerous. Working in hot conditions, in extreme cases, can lead to overheating, which can cause health problems such as prickly heat, heat cramps, fainting, heat exhaustion and heat stroke.

Fatalities have occurred from heat stroke in Canadian workplaces. Fainting and heat exhaustion are far from uncommon, and extreme discomfort among workers -- which lowers job satisfaction and may lead to reduced safety performance -- are quite common. The 224 lost-time claims accepted by WCBs across the country from 1994 to 1996 are merely the tip of the iceberg -- so to speak -- when it comes to heat as a very real hazard on the job.

To minimize the risk of illness resulting from too much heat, threshold limit values (TLVs) for heat exposure have been set by the American Conference of Governmental Industrial Hygienists (ACGIH). The guidelines are based not on air temperature, but on much more useful measurements from a heat stress monitor that measures the "wet bulb globe temperature" or WBGT, the standard for determining the true level of heat stress on a worker.

The temperature on the thermometer (or the temperature we hear reported on the radio weather reports) is a very poor predictor of the heat load that a worker will experience on the job. To get a complete picture of the heat-stress level, we need to consider a number of factors.

* The air temperature (which is the only thing a conventional thermometer measures) affects how much heat is transferred to the body from the air, and the rate at which heat can be dissipated from the body to the air.

* The relative humidity, or the amount of moisture in the air, has two major effects. First, air that is more moist is denser and holds more heat. But, more important, high humidity makes it difficult for sweat to evaporate. Less evaporation of sweat means that it’s more difficult for the body to cool itself.

* Air movement (indoor air flow or a breeze outside) has a cooling effect of its own, but, more important, it can evaporate more sweat from the body, which acts to absorb heat and cool the body.

* The amount of radiant heat to which a worker is exposed is not necessarily measured by a normal thermometer, but it can have a dramatic effect on total heat burden. Sunlight shining directly on a worker can add a great deal of heat to his or her body; so can heat radiated from a hot machine or a furnace in a foundry.

* The clothing and protective equipment worn by the worker, obviously, has a direct effect on his or her level of heat loss and heat gain. Insulating clothing tends to hold heat in and to prevent the circulation of air that would evaporate sweat and cool the body. Also, dark colours absorb more radiant heat.

* The type of work being performed determines how much heat is generated by the body. Clearly, light, sedentary work generates little heat, while active, heavy work generates a great deal.

* Acclimatization of the worker is another important factor. The body can adapt to changing environmental conditions -- we "get used to" the heat or cold -- but it takes a little time.

It is to address all of these variables that the ACGIH developed its TLVs for heat stress. The assessment of heat stress consists of analyzing three main components of the heat load actually experienced by workers: the environmental temperature conditions; the type of work being performed (whether it’s light, moderate or heavy in exertion); and the type of clothing and equipment worn by the worker. Together, these three variables translate into a recommended "work-rest regimen" that dictates how long workers can be safely exposed to a given level of heat stress.

Heat stress monitor

The WBGT index was developed by the U.S. military in the 1950s and has become the accepted standard for determining the level of heat stress on an individual. It was designed "as an attempt to predict how a human will respond to the environment," explains Mike Wurm a project engineer for Quest Technologies Inc., in Oconomowoc, Wisconsin. It takes into account the effects of humidity, air movement, air temperature and radiant heating from the sun or other sources. To measure all this, you’ll require some special equipment.

A heat stress monitor consists essentially of three thermometers that measure the three factors that determine the overall, effective environmental heat load.

* Air temperature is determined using a conventional thermometer, referred to as a dry bulb thermometer.

* A black globe thermometer is used to measure radiant heat. It consists of a conventional thermometer inserted through a rubber stopper into a hollow, six-inch-diameter copper ball coated with a flat black paint. The black metal ball absorbs radiant heat and raises the temperature inside.

* The effect of evaporation and air movement is measured with a naturally ventilated wet thermometer, called a wet bulb. This is a conventional thermometer with its bulb wrapped in an absorbent cotton wick. The wick extends 30 to 35 millimeters above the bulb of the thermometer and the lower portion of the wick is immersed in distilled water. Approximately 25 mm of moistened wick is exposed between the water and the bulb of the thermometer. The moist wick continuously provides water for evaporation and closely mimics the effect of evaporation of sweat from the body and the resulting cooling action.

Two methods are used to calculate the "wet bulb globe temperature" in the workplace. For outdoor workplaces with direct sunlight, the WBGT is calculated by taking 70 per cent of the wet bulb temperature, adding 20 per cent of the black globe reading, and 10 per cent of the air temperature (.7 web bulb + .2 black globe + .1 air temperature). For workplaces without direct sunlight, the calculation used is .7 times the wet bulb temperature plus .3 times the black globe temperature.

If the thought of applying these formulas and using this complex equipment leaves you cold, relax. You can purchase instruments that measure the temperatures directly, combining the three factors and their appropriate weighting values. No manual calculations are necessary.

Modern heat-stress monitors contain all three sensors and can instantly compute the three individual temperatures as well as the indoor and outdoor WBGT index to identify dangerous working conditions. They will also calculate the time-weighted average WBGT, which you’ll have to determine if conditions in your workplace fluctuate widely. The temperatures are displayed in Fahrenheit or Celsius on an LCD panel.

These units are portable, typically weighing about 680 grams (24 oz) and can be battery- or line-operated. Some of the more elaborate units come with data-logging capabilities and allow records to be stored and output to your computer and printer. Remote sensors are available for some types to simultaneously monitor a second or third location. With some models, the user can program work levels and clothing types, and the unit will calculate and display the length of time a worker can safely stay in the area. These work/rest regimens can be determined based on ACGIH Threshold Limit Values for Heat Stress, US Navy Physiological Heat Exposure Limits (PHEL), or ISO criteria. Prices for these monitors start at about $2,000.

Calculating the impact

Carefully following all the instructions for use of the device, you use a heat stress monitor in your workplace. It produces a reading of, say, 26 degrees C as the combined and adjusted web bulb globe temperature. (Note that 70 per cent of this reading is from the much-cooler wet bulb; in order to get a WBGT reading of 26, the air temperature will probably be in the mid to high 30s.) Now what? What do you do with this information?

You use it to calculate the allowable "work-rest regimen" for workers exposed to these conditions. The ACGIH publishes the table below.

Work-Rest Regimen Light work Moderate work Heavy work

Continuous work 30 26.7 25

75% Work, 25% Rest 30.6 28 25.9
each hour

50% Work, 50% Rest 31.4 29.4 27.9
each hour

25% Work, 75% Rest 32.2 31.1 30
each hour

For the purposes of calculating the above work levels, the ACGIH defines "light work" as sitting or standing to control machines or performing light hand or arm work. "Moderate work" is defined as walking around with moderate lifting and pushing. "Heavy work" is defined as, for example, working with a pick and shovel.

The second set of variables involves the type of clothing and equipment worn by the workers. The TLV lists four categories of clothing and corresponding WBGT correction factors. (The "correction factors" lower the maximum permissible heat exposure on the table above before the work-rest regimen is calculated.)

"Summer work uniform" lists a correction factor of zero. In other words, no adjustment to the numbers on the table is required. "Cotton coveralls" list a factor of -2 (meaning that the values have to be reduced by two when calculating work-rest schedules. "Winter uniform" has a correction of -4, while "water barrier, permeable" list a factor of -6.

So what does that WBGT reading of 26 mean? Workers are wearing light, summer clothing. This is not the first warm day of the year and your staff are pretty much used to the heat. They’re doing light to moderate assembly work in a factory. You check the work-rest regimen table and discover that continuous work is permissible. But you note that it’s close to the line, and that anyone doing heavy work under these conditions will have to have a 75-25 regimen -- a 15 minute break every hour.

Hayley Chandler is associate editor of safety purchasing for OHS CANADA Magazine.

ELLIE please note: a 2-column, square diagram is on the way. Please leave room.

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IN B.C., IT’S NOW THE LAW

British Columbia’s new Occupational Health and Safety Regulation, which includes requirements for the prevention of heat stress, took effect on April 15, 1998. If workers are at risk of heat-related disorders, the employer is required to conduct a heat stress assessment by measuring the WBGT or another acceptable measuring standard. If it is not feasible to reduce exposure below the heat exposure limits using engineering controls then the employer must provide the following:

* administrative controls such as an acceptable work-rest cycle (see main story), or,

* where it provides equally effective protection, personal protective equipment such as reflective suits or air- or water-cooled vests.

Employers must also provide heat-exposed workers with an adequate supply of cool drinking water close to the work area. Workers at risk of heat-related disorders, and their supervisors and immediate co-workers must be trained to recognize signs and symptoms of heat-related disorders, and to leave the hot environment if signs or symptoms of a heat-related disorder occur.

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BEAT-THE-HEAT PRODUCTS

It’s summertime again and most Canadians are enjoying the return of warm weather. Of course, there’s always the danger of too much of a good thing. Summer’s climate can present some potential health and safety problems, especially for those who work outdoors. But for every problem there’s a solution and a wide variety of special products are available to help us deal with summer’s down side. Here’s a look at what you might need and why.

* Drinks for everyone. Because perspiring is the body’s natural cooling mechanism, those who work in hot environments are at risk of dehydration. Water and salt lost through sweat must be replaced at least hourly. Fluid intake should equal fluid lost. Ensure that plenty of cool (10 to 15 degrees C) drinking water is available on the job site and encourage workers to drink water every 15 to 20 minutes whether or not they feel thirsty. Thirst is not an adequate indicator of the body’s need for water. Another option is to provide electrolyte replacement drinks, which are specially formulated to replace mineral salts, body fluids and sugar at optimal absorption rates. Electrolytes are minerals (magnesium, potassium, sodium and calcium) that are lost through perspiration or any form of dehydration. Electrolytes flow through muscle cells to keep them functioning normally. Even if you do not provide them to the work force on a regular basis, you may wish to keep a supply of these drinks on hand so they can be offered as part of first aid treatment for heat cramps or heat exhaustion.

* Helping workers keep their cool. You can further supplement workers’ natural cooling systems with personal cooling equipment. There many types to choose from. You can purchase hats, headbands and bandannas containing water-absorbing crystals that hold up to 1000 times their own weight. Workers soak these items in cool water and wear them on the job. Garments -- usually vests -- containing ice and gel packs are also an option. More elaborate (and expensive) systems, such as garments that contain a network of tubes through which chilled water is circulated via a battery-powered pump are most likely to be purchased for workers who work in extremely hot environments such as foundries.

* Protection from the sun. In addition to heat, you must consider the effects of the sun on those who work outdoors. The Canadian Dermatology Association (CDA) reports that in Canada alone, more than 60,000 people will be diagnosed with a common form of skin cancer this year, with exposure to ultraviolet radiation from the sun as the leading cause. To protect themselves, workers should apply sunscreen to all exposed areas of skin 15 to 30 minutes before going outdoors.

Dermatologists recommend using a broad-spectrum sunscreen with an SPF of 15 or higher. Broad-spectrum sunscreens will absorb both UVA and UVB rays. (Research suggests UVA rays age the skin and probably cause cancer.)

Sunblocks contain zinc oxide or titanium oxide. These ingredients act as physical blocks and prevent UVB and most UVA rays from reaching the skin. The level of protection provided depends on the concentration of chemicals in the preparation. The lower lip is a common site for skin cancer. Workers should use an SPF 15 sunscreen for lips (available in a stick form) and reapply it every two hours.

Workers with sensitive skin may find sunscreen cream less irritating than alcohol-based lotions. Patch testing can help avoid allergic reactions to specific brands. A water-resistant sunscreen will be better able to withstand humidity and perspiration, and therefore more effective than those that are not waterproof. An expiry date should be visible on the bottle or tube.

* Put a lid on it. Two thirds of all skin cancers appear on the head and neck and almost all of these are caused by sun exposure. Wearing a hat could significantly reduce a worker’s risk. Hats shade areas, such as the tips of the ears, bald spots and lips, which people often fail to protect with sunscreen. They also help protect the eyes and prevent absorption of radiant heat. Purchase a hat with a brim of 7.5 cm or more. Legionnaire-style hats with a back flap that shades the neck, ears and side of the face are also recommended. The hat should be made of a closely woven fabric. To test this, hold the hat up to bright sunlight or a light bulb. If light cannot get through, the hat should block the sun’s rays.

Almost all garments provide some protection from the sun because the material blocks the radiation. As with hats, the degree of protection offered varies depending on the tightness of the weave. It is also possible to purchase garments that are specially designed to reduce transmission of ultraviolet radiation. One Canadian company features hats and clothing composed of a polyester-based fibre resin treated wit h UVA and UVB sun-inhibitors. It is claimed to reduce transmission of UV rays by 89 to 92 per cent despite the fact that it is loosely woven and permits maximum breathing capability.

* Made for the shade. Ultraviolet radiation is also harmful to the eyes. Workers should wear sunglasses that have UV ray screening properties. The Canadian Standards Association standard (Z94.5-95) applies to nonprescription sunglasses that are used where safety eyewear is not specifically recommended. It classifies sunglasses as cosmetic, general purpose or special purpose. Special purpose lenses screen out 99 per cent of UVB and 60 per cent of UVA rays. They are highly recommended for those working outdoors. The CSA also certifies sunglass lenses as "robust" if they have passed special impact resistance testing. Although they would be inadequate in situations where a known occupational impact hazard exists, they are a good choice for workers who may need limited impact protection. If colour discrimination is needed, grey lenses are the best choice. A grey tint filters light evenly across the spectrum, leaving colours virtually unchanged.

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