How to keep workers safe from heat stress

Extreme heat in the workplace can have severe health effects. What kinds of PPE are needed to keep workers safe from heat stress?

How to keep workers safe from heat stress
Seasonal organizations are probably on top of [heat stress prevention]. But newer facilities, it’s not really on their radar until it gets hot out.

A bakery worker at a Weston Bakeries facility in Barrie, Ont. was working at large ovens during a heat wave in August 2001. Fifteen minutes before the end of a 12-hour shift, the worker collapsed and died of heat stroke. At the time of his death, his core body temperature was 42.5 C.

Every year, hundreds of workers in Canada suffer injuries due to heat-related causes. These injuries can have dire effects. Managers and workers should know what kinds of cooling personal protective equipment (PPE) are appropriate for their workplace. While PPE cannot be seen as the primary means of preventing injury caused by extreme heat, it is often an essential means to reduce the risk.

Heat stress is an illness caused by a buildup of body heat that is produced by the environment (air temperature, humidity, air movement and hot sources or radiation from the sun) combined with the physical exertion of the work and the clothing and equipment workers are wearing. A rise in the body’s temperature to several degrees above 37 C can cause serious and possibly fatal conditions.

Seasonal workers and others who work outside in summer are among those most at risk of heat stress, says Kelly Fernandes, occupational hygienist at Mississauga, Ont.-based Workplace Safety and Prevention Services. These include agricultural, landscape, logging, forestry and construction workers. There are also many other people who work indoors and are exposed to radiant heat from hot surfaces or substances (such as molten metals) or process heat; for example, welders and metal fabricators and others who work in steel mills, foundries, mines, oil and gas facilities and bakeries.

The health effects of heat stress include heat cramps and heat syncope, a feeling of dizziness or light-headedness. It may cause heat rash, which occurs when sweat ducts become blocked and sweat is trapped under the skin. The resulting inflammation produces symptoms such as small blisters, deep, red lumps and an intense itchy feeling.

At the extreme end, a person can suffer heat stroke, which may cause confusion, disorientation, incoherence and vomiting. The skin is dry and red hot, and sweating has stopped. Heat stroke can lead to loss of consciousness, permanent injury or death. It is often preceded by heat exhaustion, with signs that include headache, nausea, dizziness, fatigue, extreme thirst, heavy sweating and irritability.

“Workers and supervisors should be stopping that from happening. If they notice the signs and symptoms of heat exhaustion in themselves or others or if they realize they haven’t had enough water — that should be a reminder to them: I’m starting to not feel well, and they should take action,” Fernandes says.

When the risk of excessive heat exposure has been identified in the workplace, then the employer, using the hierarchy of controls, should develop a heat stress program and take preventive measures. Risk of heat stress can be reduced by:

  • working out of direct sunlight;
  • providing hydration stations and ensuring workers drink water regularly;
  • controlling the exposure temperature (reduce radiant heat from machines);
  • providing regular rest breaks in shaded or air-conditioned areas;
  • working in teams;
  • rotating tasks; and
  • reducing workload.

Before working in a very hot workplace, employers should ensure workers are acclimated to the heat, Fernandes says. Workers not used to working in high temperatures should have their exposure to heat and workload limited and then gradually increased before they are assigned regular hours and tasks. 

“You’re building up their tolerance. Acclimation takes roughly seven to 14 days, but that depends on the worker. There are worker factors that can affect acclimation. If someone is older, that can be a factor in the acclimation process,” she says.

Workers and managers should be trained to recognize the symptoms of heat stress in themselves and their co-workers. They should understand the importance of regular hydration. The company should also be prepared to provide first aid quickly, have a communication system and have trained first responders.

A heat stress assessment will, in addition to helping employers understand the heat stress risk in the workplace, help determine which preventive measures should be implemented and included in the company’s heat stress program.

When it is not possible to reduce the risk of heat stress to a safe level through engineering or administrative controls, for example, then cooling PPE should be used.

Generally, people working in hot conditions should wear garments made of cotton or other natural fibre, such as linen or silk. Outdoor workers should cover as much skin as possible, wearing long sleeves and pants. Choose clothing that is lightweight, loose fitting and comfortable.

“The air gaps help insulate you against the heat. Also wear light-coloured clothing. Dark colours absorb heat, so wear khaki, light blue or florescent yellow,” says Derek Sang, technical training manager at Nashville, Tenn.-based Bulwark Protection.

While cotton is a breathable fabric, it also absorbs moisture well and can become uncomfortable. “Once it reaches saturation, cotton stays wet. So, workers have to deal with the discomfort of dragging this wet fabric around with them during their workday,” he says.

Instead of using cotton, therefore, many cooling garments are made of “wicking” fabrics. These materials pull moisture away from the body and push it toward the surface of the fabric, where it can evaporate, thus helping to keep skin dry.

“If I’m dry, I’m going to feel cooler. If I can move the moisture from my body as it sweats quickly out into the environment, I’m going to feel cooler throughout my workday,” he says.

Wicking fabrics are used to make garments ranging from underwear and socks to shirts and coveralls.

Many wicking fabrics are made of high-tech polyester. Among other effective moisture-wicking fabrics are polypropylene, merino wool, wool, nylon and micromodal. Rayon and linen are breathable fabrics but not very effective at wicking moisture away from the skin.

A material commonly used for cooling work wear is PVA — polyvinyl acetate. PVA is extremely absorbent and can hold water for a long time. It is used to make cooling vests and cooling towels, which are worn around the neck. The user saturates the garment in water for one to two minutes, and then wrings it out before putting it on. The material is dry to the touch.

Microfibre fabrics, used to make towels, cooling neck bands and bandanas, also retain water much more effectively than regular towel material. Moreover, microfibre is soft so it’s easy to wear against the skin for long periods of time. However, cooling doesn’t last as long as with PVA. Microfibre towels are activated, like PVA, by being soaked in water.

Some manufacturers have started to combine PVA and microfibre or cotton to get both the long-lasting cooling properties of PVA and the softness of microfibre. These hybrid materials are used to make such garments as neck coolers, bandanas and headbands.

Water, air and ice are used to keep the wearer’s body temperature down. With one kind of vest, the user fills the garment with water, which stays in the vest for a few days. A vest may also be equipped to hold cooling packs made of ice or frozen phase-change material (a substance that changes form, usually between solid and liquid). The packs maintain a constant, comfortable temperature for several hours. They may be held in the vest by tape, integrated into the vest or placed in the vest pockets.

Some systems use an external device to drive cooling liquid or air through workers’ garments. Because workers are attached to an external, stationary device, their range of movement is limited.

With water-cooled garments, an external pump pushes a liquid through tubing that is sewn into the garment. The system includes a water-cooled cap and vest and water-cooled undergarments.

The personal air-conditioning vest uses a compressor and cooling (vortex) tube. The tube pumps cool, compressed air through a plasticized vest worn by the worker. The cold air circulates around the vest and reduces the temperature around the worker.

“It’s very popular with welders. We do a lot of work with the ship-building industry. When welders are in the ship’s hold, it’s quite hot in there,” says Marcus Shuter, Vortec product manager at Cincinnati, Ohio-based Vortec (an ITW company), adding that the device saves time because it requires no maintenance.

“It cools almost instantaneously. As long as you have it clipped up and the compressed air going through it, it’s cooling. So, there’s no downtime.”

In direct sun, wearing a hat prevents overheating. Cooling pads and skull caps can be worn inside a hard hat, and a flexible shade can be attached to a hard hat brim to protect the neck and eyes from the sun. Outdoor workers should wear polarized sunglasses. If possible, choose work gloves made of material that is lightweight, breathable and light coloured.

In some facilities, aluminum coverings are needed to provide protection against excessive radiant heat. These specialized garments, including aprons, jackets, suits and gloves, must cover the worker from neck to feet. Aluminized heat-reflective clothing can reflect up to 95 per cent of radiant heat.  

Many standard kinds of PPE are not practical for workers who, due to the presence of electric-arc, flash fire, combustible dust and electrical hazards also need flame-resistant clothing. For manufacturers, the problem has been to provide a worker with personal clothing or equipment that will cool the worker but, at the same time, not put the worker at increased risk.

“That challenge is not easily solved. Unfortunately, if there is a high-energy, thermal event, like an arc flash or a flash fire, cooling PPE like the cold-gel packs can melt and add to the injury. The harness that holds the gel packs is typically not flame resistant, it doesn’t have arc rating, so it could transfer injury to the wearer,” Sang says.

“Things like bandanas [and] cooling cloths are made from microfibre cloth — they’re also typically going to melt, drip and add to the injury.”

However, he adds, there are fabrics that combine FR and moisture-wicking properties. They are used to make FR work garments such as coveralls and shirts. Otherwise, where heat and flame hazards co-exist, employers should try to reduce worker heat exposure through safer work practices and not rely on PPE.

Various organizations have developed calculation tools to help managers and workers reduce or prevent heat stress. For example, the Montreal-based Robert-Sauvé Research Institute for Occupational Health and Safety (IRSST) provides access to three computer-based tools on its website.

Two of these tools assist in the calculation of the “alternate work-rest regimen” needed when various hot work conditions apply, says Capucine Ouellet, occupational hygienist at the IRSST. One of them estimates the regimen according to the occupational health and safety regulation of Quebec. The other estimates the work-rest regimen based on the exposure limits recommended by the Cincinnati, Ohio-based American Conference of Governmental Industrial Hygienists (ACGIH), which sets maximum exposure limits for various workplace hazards. Most jurisdictions in Canada follow ACGIH guidelines when setting exposure limits.

“It gives you a precise number for rest time. It tells you how many minutes you need to rest per hour of work, considering the level of heat stress. This is a minimum,” Ouellet says.

The third computer-based tool on the IRSST website calculates the “corrected” air temperature (CAT). Users enter information regarding air temperature, relative humidity, exposure conditions and workload. The tool calculates a “corrected” or actual temperature and indicates the level of heat stress, while recommending preventive measures (including hydration frequency) appropriate for that risk level.

In the lower-risk green zones, the recommendations are more about preparing in case the heat stress rises. “That’s because heat stress changes every hour. In the morning, you can be in the green zone. In the early afternoon, you can be in the yellow [higher-risk] zone, though still doing the same work — because the temperature rises as the day goes on. That’s true for outdoor work and sometimes true also for indoor work, as in kitchens or foundries,” Ouellet says.

“This CAT tool is very easy, friendly. Everybody can use it in the field. It’s good for workers and employers.”

All companies should begin planning for heat stress before temperatures climb. Many companies, Fernandes says, don’t start considering the risk of heat stress until the summer is well underway.

“Seasonal organizations are probably on top of it. But newer facilities, it’s not really on their radar until it gets hot out,” she says. “That’s when assessments happen, and they start creating their heat stress program for the next year.”