Risk factors of heat stress that should be considered in assessing heat stress include:

1. Personal
2. Environmental
3. Job-related

Personal Risk Factors of Heat Stress

Physical Conditions

It is difficult to predict just who will be affected by heat stress and when?

because individual susceptibility varies.

There are, however, certain physical conditions that can reduce the body’s
natural ability to withstand high temperatures:

Weight: Workers who are overweight are less efficient at losing heat.

Poor physical condition: Being physically fit aids your ability to cope with the increased demands that heat places on your body.

Previous heat illnesses: Workers are more sensitive to heat if they have
experienced a previous heat-related illness.

Age: As the body ages, its sweat glands become less efficient.

Workers over the age of 40 may therefore have trouble with hot environments.

Acclimatization to the heat and physical fitness can offset some age-related problems.

Heart disease or high blood pressure: In order to pump blood to the skin and cool the body, the heart rate increases.

This can cause stress on the heart.

Recent illness: Workers with recent illnesses involving diarrhea, vomiting, or fever have an increased risk of dehydration and heat stress because their bodies have lost salt and water.

Alcohol consumption: Alcohol consumption during the previous 24
hours leads to dehydration and increased risk of heat stress.

Medication: Certain drugs may cause heat intolerance by reducing sweating or increasing urination.

People who work in a hot environment should consult their physician or pharmacist before taking medications.

Lack of Acclimatization

When exposed to heat for a few days, the body will adapt and become more efficient in dealing with raised environmental temperatures.

The name of this process is Acclimatization.

Acclimatization usually takes six to seven days.

Benefits include:

— lower pulse rate and more stable blood pressure
–more efficient sweating (causing better evaporative cooling)
— improved ability to maintain normal body temperatures.

Acclimatization may be lost in as little as three days away from work.

People returning to work after a holiday or long weekend—and their supervisors— should understand this.

Workers should be allowed to gradually re-acclimatize to work conditions.

Environmental Risk Factors of Heat Stress

Environmental factors such as ambient air temperature, air movement, and relative humidity can all affect an individual’s response to heat.

The body exchanges heat with its surroundings mainly through radiation and sweat evaporation.

The rate of evaporation is influenced by humidity and air movement.

Radiant Heat

Radiation is the transfer of heat from hot objects through air to the body.

Working around heat sources such as kilns or furnaces will increase heat stress.

Additionally, working in direct sunlight can substantially increase heat stress.

A worker is far more comfortable working at 24°C under cloudy skies than working at 24°C under sunny skies.

Humidity

Humidity is the amount of moisture in the air.

Heat loss by evaporation is hindered by high humidity but helped by low humidity.

As humidity rises, sweat tends to evaporate less.

As a result, body cooling decreases and body temperature increases.

Air Movement

Air movement affects the exchange of heat between the body and the environment.

As long as the air temperature is less than the worker’s skin temperature,

increasing air speed can help workers stay cooler by increasing both the rate of evaporation and the heat exchange between the skin surface and the surrounding air.

Job-Related Risk Factors of Heat Stress

Clothing and Personal Protective Equipment (PPE)

Heat stress can be caused or aggravated by wearing PPE such as fire- or chemical-retardant clothing.

Coated and non-woven materials used in protective garments block the evaporation of sweat and can lead to substantial heat stress.

The more clothing worn or the heavier the clothing, the longer it takes
evaporation to cool the skin.

Remember too that darker-coloured clothing absorbs more radiant heat than lighter-coloured clothing.

Workload

The body generates more heat during heavy physical work.

For example, workers shoveling sand or laying brick in hot weather generate a tremendous amount of heat

and they are at risk of developing heat stress without proper precautions.

Heavy physical work requires careful evaluation even at temperatures as low as 23°C to prevent heat disorders.

This is especially true for workers who are not acclimatized to the heat.

Evaluating Risk Factors of Heat Stress

To prevent heat stress, the World Health Organization (WHO) have determined that workers should not be exposed to environments that would cause their internal body temperature to exceed 38°C.

The only true way of measuring internal body temperature is rectally (oral or inner ear measurements are not as accurate).

As an alternative, the American Conference of Governmental Industrial Hygienists (ACGIH) has developed a method of assessing heat stress risk based on a wet bulb globe temperature (WBGT) threshold.

This method of assessment involves the three main components of the heat burden experienced by workers:

1. Thermal environment
2. Type of work
3. Type of clothing

Thermal Environment

• air temperature

• radiant heat (heat transmitted to the body through the air from hot objects such as boilers or shingles heated by the sun)

• cooling effects of evaporation caused by air movement (humidity).

To measure WBGT, a heat stress monitor consisting of three types of thermometers is required:

1) A normal thermometer called a dry bulb thermometer is used to measure air temperature.

2) Radiant heat is measured by a black bulb globe thermometer.

This consists of a hollow, 6-inch diameter copper ball painted flat black and placed over the bulb of a normal thermometer.

3) A wet bulb thermometer measures the cooling effect of evaporation caused by air movement (wind or fan).

It consists of a normal thermometer wrapped in a wick kept moist at all times.

As air moves through the wet wick, water evaporates and cools the thermometer in much the same way that sweat evaporates and cools the body.

Heat stress monitors currently available calculate WBGT automatically.

The equipment required and the method of measuring WBGT can be found in the ACGIH booklet TLVs® and BEIs®:

Threshold Limit Values…Biological Exposure Indices.

The booklet also outlines permissible exposure limits for heat stress.

Older instruments, however, require calculation by the operator.

Calculation depends on whether sunlight is direct (outdoors) or not (indoors).

Working outdoors in direct sunlight

For work in direct sunlight,WBGT is calculated by taking 70% of the wet bulb temperature,

adding 20% of the black bulb temperature, and 10% of the dry bulb temperature.

WBGT (out) = [70% (0.7) x wet bulb temperature] + [20% (0.2) x black bulb
globe temperature] + [10% (0.1) x dry bulb temperature]

Working indoors (no sunlight)

For work indoors or without direct sunlight,

WBGT is calculated by taking 70% of the wet bulb temperature and adding 30% of the black bulb temperature.

WBGT (in)= [70% (0.7) x wet bulb temperature] + [30% (0.3) x black bulb globe temperature]

For Example

Suppose it’s a bright sunny day and a crew of roofers is working 20 feet above ground.

Our assessment yields the following readings:

Wet bulb temperature (cooling effects of evaporation) = 20°C

Black bulb globe temperature (radiant heat) = 36°C

Dry bulb temperature (air temperature) = 33°C

Using the formula for work in direct sunlight, we calculate as follows:

WBGT = (0.7 x wet bulb temperature) + (0.2 x black bulb globe temperature) + (0.1 x dry bulb temperature)

= (0.7 x 20) + (0.2 x 36) + (0.1 x 33) = 14 + 7.2 + 3.3

WBGT (outdoors) = 24.5 °C

Type of Work

The second factor in assessing heat stress is the type of work being performed.

Following are the four categories, with some examples of each:

Low

• Using a table saw

• Some walking about

• Operating a crane, truck, or other vehicle

• Welding

Heavy work

• Laying brick

• Walking with moderate lifting or pushing

• Hammering nails

• Tying rebar

• Raking asphalt

• Sanding drywall

Very Heavy Work

• Carpenter sawing by hand

• Shoveling dry sand

• Laying block

• Ripping out asbestos

• Scraping asbestos fireproofing material

• Shoveling wet sand

• Lifting heavy objects

Work/Rest Schedules

The WBGT can be used to determine work/ rest schedules for personnel under various conditions.

Knowing that the WBGT is 24.5°C in the example above,

you can refer to Table below and determine that workers accustomed to the heat (“acclimatized”),

This table is intended as an initial screening tool to evaluate whether a heat stress situation may exist.

These values are not intended to prescribe work and recovery periods.

wearing summer clothes, and doing “heavy” work can perform continuous work (100% work).

Suppose work is being performed indoors at a pulp and paper mill under the following conditions:

• Workers are wearing cloth coveralls.

• Boilers are operational.

• Work load is moderate.

• General ventilation is present.

Exemple

Our assessment yields the following readings:

Wet bulb temperature (cooling effects of evaporation) = 23°C

and Black bulb globe temperature (radiant heat) = 37°C

Dry bulb temperature (air temperature) = 34°C

Using the formula for work indoors, we calculate as follows:

WBGT = (0.7 x wet bulb temperature) + (0.3 x black bulb globe temperature)
= (0.7 x 23) + (0.3 x 37) = 27.2°C

Addition for cloth coveralls (Second Table ) = 0

WBGT (indoors) = 27.2°C

Referring to previous table, we determine that workers accustomed to the heat, wearing cloth coveralls, and performing “moderate” work can work.

The WBGT must never be used as an indicator of safe or unsafe conditions.

It is only an aid in recognizing heat stress.

The ultimate assessment and determination of heat stress must lie with the
individual worker or co-worker trained to detect its symptoms.

Supervisors must allow individual workers to determine if they are capable of working in heat.

The next table is intended for use as a screening step only.

Detailed methods of analysis are fully described in various technical and reference works.

Type of Clothing

Free movement of cool, dry air over the skin maximizes heat removal.

Evaporation of sweat from the skin is usually the major method of heat
removal.

WBGT-based heat exposure assessments are based on a traditional summer work uniform of long-sleeved shirt and long pants.

With regard to clothing, the measured WBGT value can be adjusted according to the following table:

These values must not be used for completely encapsulating suits, often called Level A.

and Clothing adjustment factors cannot be added for multiple layers.

The coveralls assume that only modesty clothing is worn underneath, not a second layer of clothing.

Notes

• WBGT values are expressed in °C. WBGT is NOT air temperature.

• WBGT-based heat exposure assessments are based on a traditional summer work uniform of long-sleeved shirt and long pants.

• If work and rest environments are different, hourly time-weighted averages (TWA) should be calculated and used.

• TWAs for work rates should also be used when the demands of work vary within the hour.

• Because of the physiological strain produced by very heavy work among less fit workers, the table does not provide WBGT values for very heavy work in the categories 100% Work and 75% Work; 25% Rest.

• Use of the WBGT is not recommended in these cases. Detailed and/or physiological monitoring should be used instead.

• Consult the latest issue of TLVs® and BEIs®:

Threshold Limit Values® and Biological Exposure Indices®,

published by the American Conference of Governmental Industrial Hygienists, for guidance on how to properly measure, interpret, and apply the WBGT.

Because many workplaces are transient and variable in nature it may not be practical to measure WBGT.

It’s therefore reasonable to ask if there are other ways to evaluate heat stress risk.