Heat stress is a danger to workers and a potential source of liability under the OSHA General Duty Clause, Sec. 5(a)(1) of the Occupational Safety and Health Act. OSHA doesn’t have a heat stress standard. However, an OSHA Directive (TED 01-00-015) which instructs OSHA inspectors how to determine whether to issue citations for heat stress violations sets out the 5 kinds of measures employers are supposed to implement to control heat stress dangers.
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Acclimatization refers to the human body’s method of adapting to heat exposure. After a acclimatization, the same activity produces fewer cardiovascular demands. The worker sweats more efficiently (causing better evaporative cooling), and is thus better able to maintain normal body temperatures.
According to the Directive, a properly designed and applied acclimatization program decreases the risk of heat-related illnesses. Such a program basically involves exposing employees to work in a hot environment for progressively longer periods. NIOSH (1986) says that, for workers who have had previous experience in jobs where heat levels are high enough to produce heat stress, the regimen should be 50% exposure on day 1, 60% on day 2, 80% on day 3 and 100% on day 4. For new workers similarly exposed, the regimen should be 20% on day 1, with a 20% increase in exposure each additional day.
2. Fluid Replacement
Ample supplies of cool (50°-60°F) water or any cool liquid (except alcoholic beverages) should be made available to workers in or near the work area. Workers should be encouraged t to drink small amounts frequently, e.g., one cup every 20 minutes.
3. Engineering Controls
The Directive lists 7 general types of engineering controls that can be used to reduce heat stress dangers in hot work environments:
General ventilation can be used to dilute hot air with cooler air (generally from the outside). This technique clearly works better in cooler climates. A permanently installed ventilation system usually handles large area or entire buildings. Portable or local exhaust systems may be more effective or practical in smaller areas.
Air treatment/air cooling differs from ventilation because it reduces the temperature of the air by removing heat (and sometimes humidity) from the air.
Air conditioning cools the air but is expensive to install and operate, the Directive acknowledges. An alternative is the use of chillers to circulate cool water through heat exchangers over which air from the ventilation system is then passed; chillers are more efficient in cooler climates or in dry climates where evaporative cooling can be used.
Local air cooling can be effective in reducing air temperature in specific areas. Two methods have been used successfully in industrial settings. One type, cool rooms, can be used to enclose a specific workplace or to offer a recovery area near hot jobs. The second type is a portable blower with built-in air chiller. The main advantage of a blower, aside from portability, is minimal set-up time.
Convection can reduce heat stress by using fans to increase the air flow (assuming air temperature is less than the worker’s skin temperature). Changes in air speed can help workers stay cooler by increasing both the convective heat exchange (the exchange between the skin surface and the surrounding air) and the rate of evaporation. Because this method doesn’t actually cool the air, any increases in air speed must impact the worker directly to be effective, the Directive notes.
Heat conduction methods include insulating the hot surface that generates the heat and changing the surface itself.
Simple engineering controls used to reduce radiant heat, i.e. heat coming from hot surfaces within the worker’s line of sight, can include shields. Surfaces that exceed 35°C (95°F) are sources of infrared radiation that can add to the worker’s heat load. Flat black surfaces absorb heat more than smooth, polished ones. Having cooler surfaces surrounding the worker assists in cooling because the worker’s body radiates heat toward them.
With some sources of radiation, such as heating pipes, it’s possible to use both insulation and surface modifications to achieve a substantial reduction in radiant heat. Instead of reducing radiation from the source, shielding can be used to interrupt the path between the source and the worker. Polished surfaces make the best barriers, although special glass or metal mesh surfaces can be used if visibility is a problem.
Shields should be located so that they do not interfere with air flow, unless they are also being used to reduce convective heating. The reflective surface of the shield should be kept clean to maintain its effectiveness.
4. Administrative Controls & Work Practices
The next set of measures set out in the Directive deal with how the actual work is conducted.
The first good work practice the Directive cites is training. Unless all employees understand the reasons for using new, or changing old, work practices, the chances of such a program succeeding are greatly reduced. The Directive then cites NIOSH (1986) listing the components a heat stress training program should include:
Another work practice is to schedule hot jobs for the cooler part of the day, and routine maintenance and repair work in hot areas for the cooler seasons of the year.
Administrative controls that can be used to reduce heat stress cited by the Directive:
5. Worker Monitoring
Every worker who works in extraordinary conditions that increase the risk of heat stress should be personally monitored. These conditions include wearing semipermeable or impermeable clothing when the temperature exceeds 21°C (69.8°F), working at extreme metabolic loads (greater than 500 kcal/hour), etc.
Personal monitoring can be done by checking the heart rate, recovery heart rate, oral temperature, or extent of body water loss.
To check the heart rate, count the radial pulse for 30 seconds at the beginning of the rest period. If the heart rate exceeds 110 beats per minute, shorten the next work period by one third and maintain the same rest period.
The recovery heart rate can be checked by comparing the pulse rate taken at 30 seconds (P1) with the pulse rate taken at 2.5 minutes (P3) after the rest break starts.
Oral temperature can be checked with a clinical thermometer after work but before the employee drinks water. If the oral temperature taken under the tongue exceeds 37.6°C, shorten the next work cycle by one third.
Body water loss can be measured by weighing the worker on a scale at the beginning and end of each work day. The worker’s weight loss should not exceed 1.5% of total body weight in a work day. If a weight loss exceeding this amount is observed, fluid intake should increase.
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