Heat Stress – Cause, Health Effects, Assessment and Controls 

Heat stress is the average heat load an industrial worker might be subjected to as a result of the combined effects of work-related processes and atmospheric conditions such as humidity, air temperature, radiant heat exchange, air movement, and clothing requirements. It is most commonly caused by prolonged exposure to high temperatures and can be exacerbated by high humidity and physical exertion. Some industries that are particularly susceptible to heat stress include construction, mining, agriculture, and manufacturing.

Heat exposure can cause minor pain to a potentially fatal illness known as heat stroke. Mild to moderate heat stress can have a negative impact on performance and safety, and Heat-related ailments become more likely when heat exposure exceeds human tolerance limits. Some of the symptoms of heat stress include dizziness, fatigue, and muscle cramps.

Workplace heat stress management is essential due to the health effects faced by industrial workers. Heat stress is a common problem in many industries in Australia, particularly in those involved in physical work or working in hot environments.

Heat Strain

Heat strain is the human body’s overall reaction to heat stress. These reactions are aimed at eliminating excess heat from the body. The body’s natural cooling system, which includes sweating and blood flow to the skin, cannot dissipate the heat efficiently. As a result, the body’s core temperature rises. In severe cases, heat strain can progress to heat exhaustion or heat stroke.

Workers vulnerable to Heat Stress

Some workers may be more susceptible to heat stress than others.

These workers include:

• the dehydrated employees
• not adapting to workplace heat levels
• those who are not physically fit.
• having a limited aerobic capacity as determined by maximum oxygen consumption, and
• overweight workers (BMI is required to be below 24-27);
• elder in age, i.e., above 50 years;
• and those suffering from health disorders like diabetes, heart disease, circulatory, and skin disorders, hypertension, etc.

Illnesses related to heat

Illnesses related to heat stress are the by-product of employees being exposed to extreme heat and an environment that causes Acute Illnesses. Incorrect treatment of increased temperature settings can result in a variety of acute diseases ranging from:

• prickly heat
• heat cramps – they are very painful but can be cured if treated in time.
• Heat exhaustion – another severe disease that can cause prostration.
• Heat syncope – exhaustion leads to heat syncope or fainting.
• Heat stroke – Heat stroke is the most serious of the heat-related disorders that require treatment since it can be fatal or result in irreparable tissue damage. The most efficient technique of eliminating heat from the body has been discovered to be whole-body immersion in a cold/ice water bath (Casa et al., 2007).

Heat Stress Risk Assessment

Use a structured assessment since it gives you the flexibility to adapt it to each unique situation. AIOH recommends a three-stage risk assessment for assessing heat exposure in a way that may be used in a variety of situations where there is a chance of heat stress.

Three-Stage Approach

• The first level, or the basic thermal risk assessment, is primarily intended to be a qualitative risk assessment that may be administered, used, and interpreted with limited technical skills. If a possible problem is identified in the first stage, proceed to the second level of assessment to allow for a more thorough analysis of the situation and general surroundings.
• The second stage of the procedure shifts towards a quantitative risk assessment and necessitates the measurement of a variety of environmental and human elements, as the goal is to collect information about conditions to be able to perform a detailed analysis.
• The third phase, which is a more quantitative risk strategy, needs physiological monitoring of the individual. Third-stage assessment is restricted to someone with specialist knowledge and skills.

It should be noted that different risk assessment levels call for various levels of technical proficiency. The proper tool must be chosen and used for the appropriate scenario and assessor’s skill level.

Stage 1 – Basic Thermal Risk Assessment

The “Basic Thermal Risk Assessment” procedure is a recommended protocol for the level 1 assessment.

Simple tools can be used by employees or technicians to provide advice, as well as a training tool to demonstrate factors that impact heat stress. It is not meant to be a final evaluation tool, and this risk assessment takes into account a variety of elements that might contribute to heat stress, such as acclimatisation, job demands, location, clothing, and other considerations.

Apparent temperature (Steadman, 1979) can be used to estimate basic thermal risk. Apparent temperature measures how hot or cold it feels to the human body based on the combined effects of temperature, humidity, and wind speed. It is also known as the “feels like” temperature, as it takes into account factors that can affect how the body perceives the ambient temperature. For example, high humidity can make it feel much hotter than the actual temperature, while a strong wind can make it feel cooler. The apparent temperature is calculated using a formula that considers these factors and provides a more accurate representation of how hot or cold it feels to the body. Weather forecasters often use it to help people plan for outdoor activities and make decisions about what to wear.

A checklist with factors that are known to cause heat stress is used to identify and rate a workplace/work environment.

Stage 2 Assessment – Use of Rational Indices

Once the “Basic Thermal Risk Assessment” reveals that the risk is unacceptable, a detailed stage 2 assessment is recommended. If impermeable clothing is worn during some tasks, it is recommended to proceed with stage 3 risk assessment.

Stage 2 assessment requires that a minimum set of data is collected to perform an assessment. Data should be collected about the following:

• clothing type,
• airspeed,
• air temperature,
• humidity,
• work posture,
• length of exposure, and
• globe temperature.

It is recommended that detailed rational analysis should follow ISO 7933 – Predicted Heat Strain (PHS) or Thermal Work Limit (TWL), or other indices may also be used.

Thermal Work Limit (TWL)

Thermal Work Limit (TWL) is defined as the limiting (or maximum) sustainable metabolic rate that well-hydrated, acclimatised individuals can maintain in a specific thermal environment within a safe deep body core temperature (< 38.2 °C or 100.8 °F) and sweat rate (< 1.2 kg or 2.6 lb per hour). The index is designed for self-paced workers and does not rely on the estimation of actual metabolic rates, a process that is difficult and subject to considerable error.

The measure is tailored to self-paced employees and does not rely on estimates of real metabolic rates.

Work areas are assessed and classified using a metabolic heat balance calculation, with dry bulb, wet bulb, and air movement used to calculate air-cooling power (W.m-2 ).

The TWL uses the following five environmental parameters:

• Dry bulb temperature
• Wet bulb temperature
• Globe temperatures
• Wind speed, and
• Atmospheric pressure.

With the addition of clothing factors, it is possible to anticipate a safe maximum continuously sustained metabolic rate (W.m-2) for the situations under consideration.

A thermal strain metre can be used to determine the features of this index. When combined with this instrument, the TWL is a simple rational index that may be used to assess work constraints caused by a hot working environment. As previously said, because it is a logical index that evaluates a wide variety of influencing elements, it may also be used to identify controls and their efficacy.

Stage 3 Assessment – Physiological Monitoring

Physiological monitoring can be performed using a range of different technologies, such as sensors, wearable devices, and imaging techniques.

It is anticipated that the suggested TWL evaluation approach will be applicable in most common day-to-day industrial circumstances.

The physiological changes that occur during hot conditions and/or high workloads are changes in Core Temperatures, Sweat rate, and Heart rate.

Core Temperature: Body core temperature has been the most often used research technique in the field of heat stress.

Heart Rate: Heart rate has long been accepted as an effective measure of strain on the body and features.

Controls Measures

1. Heat Acclimatisation

Heat acclimatization is the process by which the body adjusts to hot weather conditions. This process typically involves a number of physiological changes that help the body to cope with the increased heat and maintain a stable internal body temperature. Some of these changes include increased blood flow to the skin, increased sweating, and changes in the body’s salt and water balance.

The benefits of heat acclimatisation are as follows:

• More finely tuned sweating reflexes, with increased sweat production rate at lower electrolyte concentrations;
• Lower skin and rectal temperatures than it was at the start of the experiment;
• Blood pressure that is more steady and well-regulated, with lower pulse rates;
• Enhanced safety and productivity;
• Decreased resting heart rate in hot weather;
• Lowered resting core temperature;
• Rise in plasma volume;
• Change in the composition of sweat;
• Lowering the sweating threshold; and
• Enhanced sweating efficiency.

2. Ventilation

Natural or forced mechanical ventilation is utilised to remove or dilute the current hot air at a workplace with cooler air. It will also play a role in situations when relative humidity is high, allowing for more effective perspiration evaporation.

There are three different types of systems used:

1. a) Forced Draft – air is blasted into space, causing exhaust air to escape.
2. b) Exhaust – air is extracted from a compartment or vessel, enabling passive air to enter through another opening.
3. c) Push-pull – a hybrid of the two processes described above in which one fan exhausts air through one entrance while another forces new air in via another.

3. Radiant Heat

Radiant heat from a wide range of sources may be managed in a variety of ways.

Insulation, shielding, and modifying surface emissivity are the three most widely employed approaches.

Shielding is a simple and efficient method of protecting against radiant heat.

When working in the sun, huge umbrellas and portable shade structures have also been shown to be reasonably inexpensive and effective measures.

4. Elimination or Substitution Controls

Hot work should be arranged to avoid the warmest portion of the day and, if possible, completed during night shifts.

• Light-coloured or reflecting materials should be used for walls and roof constructions.
• Structures should be constructed to allow for enough air movement. This may be accomplished by strategically placing windows and shutters and designing the roof to create ‘chimney effects.’ This will aid in the removal of heat from the building.
• Insulate the walls and roofing.

5. Engineering Controls

Containers and pipework involved with hot operations should be insulated and encased to reduce heat input into the workplace.

• In high humidity locations, such as northern Australia, more air must be moved, which necessitates the use of fans to improve air movement or, in severe circumstances, cooled air from ‘chiller’ units.
• In situations where radiated heat from a process is an issue, insulating or reflecting barriers can be utilised to absorb or redirect radiant heat. These might be fixed structures or moveable screens.
• Moving heated processes away from high-traffic areas.
• Dehumidifying the air to boost evaporative cooling. Steam leaks, open process containers, or standing water may artificially raise a structure’s humidity level.

Use mechanical assistance to lower the individual’s metabolic effort.

6. Administrative controls

The administrative controls may be used in conjunction with environmental controls if the latter is unable to achieve the requisite levels of remediation to decrease risk to an acceptable level.

According to the Health Protection Agency, the top priority system during heat stress exposures should be self-assessment (HPA). This enables well-qualified persons to use their judgement to decrease the possibility of overexposure to heat stress. An individual’s physical health might fluctuate from day to day, regardless of how thoroughly a monitoring system is employed.

a) Training

Training is an essential component of any health management program. In terms of heat stress, it should be carried out for all individuals who are likely to be associated with:

• High temperatures;
• Physically intensive labour at high temperatures; or
• The employment of impermeable protective equipment.

The following topics should be covered in training:

1. Heat exposure mechanisms;
2. Potential heat exposure situations;
3. Recognition of predisposing factors;
4. Fluid intake importance;
5. The nature of acclimatisation;
6. Effects of using alcohol and drugs in hot environments;
7. Early recognition of symptoms of heat illness;
8. Prevention of heat illness;
9. First aid treatment of heat-related illnesses;
10. Self-assessment

b) Self-assessment

Self-assessment is an important component of training for persons who may be subjected to heat stress. Individuals with a proper understanding of signs and symptoms will be able to detect the development of a heat disease in its early stages. This could simply entail taking a short break and drinking some water.

c) Fluid Replacement

Fluid replacement is critical when working in hot surroundings, especially when there is also a labour (metabolic) load. Moderate dehydration is frequently accompanied by a thirst feeling, which, if neglected, can lead to severe levels of dehydration (>5% of body weight) within 24 hours. Even in conditions when water is readily available, most people practically never replenish their sweat loss, resulting in a slightly negative total body water balance (BOHS, 1996).

d) Rescheduling heat-related work

It might be possible in certain cases to reschedule heated-related work for a cooler time of day. This is especially true for planned maintenance or normal process modifications. While this is not always possible, especially during maintenance or unplanned outages, some projects may require it.

e) Work and Rest regimes

Permitted exposure times (AET) or stay times are a difficult matter. It is affected by a variety of elements, including metabolism, clothing, acclimatisation, and overall health, in addition to ambient circumstances. The Wet Bulb Globe Temperature is one of the most well-known 77 systems in use (WBGT).

It must be emphasised that these restrictions are simply suggestions and not official safe/unsafe limits. They are also inapplicable to personnel dressed, not impermeable garments.

f) Pre-employment Health Assessment

Pre-employment health screening should be considered to identify persons at risk of systemic heat sickness or who work in activities with high heat stress exposures. ISO 12894 specifies the medical management of patients subjected to excessive heat.

g) Clothing

The type of clothing significantly affects workers’ exposure to humidity and heat stress. Hence, it is essential that organisations provide work uniforms to workers that have a proper cooling mechanism.

As per the proposed structured assessment protocol in stage 1, the requirements for the degree of cooling offered to workers completely dressed in summer work clothing have been established (lightweight pants and shirt). Other garments functioning as an extra insulating layer or further decreasing ambient air flowing freely over the skin are modifications to that cooling rate.

Convective heating or cooling is decided by the temperature difference between the worker’s skin and the air’s velocity of airflow. In almost all practical scenarios, air movement leads to cooling through sweat evaporation.

Because the air above the skin is saturated and not being exchanged, moisture removal from the skin surface may be limited, limiting evaporative cooling.

h) Personal Protective Equipment

When environmental or administrative controls are insufficient, it is occasionally required to utilise personal protective equipment (PPE) as an auxiliary to the prior techniques.

In current times, many different devices and systems are available, and they generally fall into one of the categories given below:

A) Air Circulating Systems

B) Reflective Systems

C) Ice Cooling Systems

D) Liquid Circulating Systems

Air Cooling System

A vortex tube cooling device, which is commonly used in air circulation systems, divides compressed air into two streams, one hot and one cold. There are no moving parts or electrical needs, and commercially available devices using factory compressed air at 690 kPa may provide cooling capacities of up to 1,760 W. Depending on the size of the vortex tube, it may be used on a huge volume, such as a vessel, or it can be used as a personal system coupled to a belt and feeding a helmet or vest. The cooled air can be inhaled using a breathing helmet, identical to those used by spray painters, abrasive blasters, or a cooling vest.

Reflective Clothing

Reflective clothing is used to minimise an individual’s radiant heat burden. It functions as a barrier between the person’s skin and the heated surface, deflecting infrared radiation away from the skin. The most typical reflective clothing configuration is an aluminised surface attached to a base fabric. Asbestos was commonly used in the past. However, materials such as Kevlar®, rayon, leather, or wool have now replaced them. The choice of the base material is also influenced by the needs of the specific environment (i.e., thermal insulation, weight, strength, etc.). The wardrobe style is also determined by employment.

Ice Cooling Systems

Contrary to the traditional ice-cooling clothing that utilised placing ice in an insulating garment near the skin to transmit heat away technique of ice cooling cools the blood in the veins near the skin’s surface, assisting in the reduction of core temperature. The improved mobility provided by the ice system is one of its primary advantages. It is also far less expensive than air or liquid circulation systems. The contact temperature has been a regular issue among ice garment wearers. Some have speculated that the coolness of the ice may cause vasoconstriction of blood vessels, reducing efficacy.

Liquid Circulating Systems

These systems operate on the heat dissipation principle, moving heat from the body to the liquid and subsequently to the heat sink (which is usually an ice water pack). They must be worn in close touch with the skin. The clothing ensemble may consist of a shirt, trousers, and hood linked with tiny capillary tubes through which cooled liquid is circulated. The pump systems are powered by a battery pack worn on the hip or back or by an “umbilical cord” connected to a distant cooling unit. The modular design without the cord enables greater mobility.

If you wish Anitech’s Occupational Hygienists to guide you in implementing effective practices for your work sites and help protect the health of your employees, do contact us!

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