Mitigating Heat Stress with Large-Diameter Ceiling Fans

Date: 2024-10-25 17:18:00

By Christian Taber, BEMP, HBDP, CEM

(Editor’s note: This article was published in the 2024 edition of AMCA inmotion magazine as "The Human Cost of Heat Stress and the Role of Large-Diameter Ceiling Fans in Mitigating Risk.")

July 21-24, 2024, were Earth’s four hottest days on record. This was not a mere heat wave, the result of a rare and relatively short-lived combination of meteorological phenomena, but, rather, part of a 13-months-long streak of unprecedented warmth.¹ As temperature records fall, heat stress is on the rise, creating health and safety challenges for communities, employers, and more.

Heat stress is a physiological condition caused by exposure to extreme heat, be it outdoors or indoors. It occurs when the body’s temperature-regulation mechanisms are overwhelmed and can lead to health outcomes ranging from heat exhaustion and heat stroke to organ failure and even death.

American Medical Association (AMA) data show heat exposure was the underlying cause or a contributing cause in more than 2,300 deaths in the United States from 2019 to 2023, an increase in number of deaths per million people of approximately 50 percent compared with the previous 40 years.² Given there is no federal law requiring heat-related illnesses to be reported to public health agencies and the fact symptoms often go unrecognized or are misidentified, the actual number of deaths likely is much higher.

To provide a more complete picture of heat-related illness in the United States, the U.S. Department of Health & Human Services (HHS) Office of Climate Change and Health Equity (OCCHE) in 2023 launched the Heat-Related EMS Activation Surveillance Dashboard. Updated weekly, the tool tracks emergency-medical-services (EMS) responses to people experiencing heat-related emergencies in pre-hospital settings. The fact the dashboard was modeled after a tool designed to track and prevent opioid overdoses is a clear signal extreme heat officially has become a national health crisis.

People seeking shelter from the Phoenix area’s 15th consecutive day of temperatures exceeding 110°F (43°C) rest at the First Congregational United Church of Christ cooling center July 14, 2023. The church opened its doors, providing water, food, and refreshments to residents seeking relief from the heat. Credit: Brandon Bell/Getty Images

Heat Balance

Normal core body temperature is approximately 98.6°F (37°C).³ Heat illness occurs when the body is unable to maintain a balance between heat generation and heat loss and core body temperature rises. Heat stroke occurs when core body temperature reaches 104°F (40°C).

The following equation shows heat generation as three separate factors: basal metabolism (M_b), posture (M_p), and activity (M_a):⁴

M = M_b + M_p + M_a

Figure 1 shows the heat-generation breakdown for a seated occupant doing light work, which equates to approximately 1.1 Met, or 65 W/m². Also shown is balancing heat loss for two scenarios:

Heat Loss 1: 75°F (23.9°C) air dry bulb, mean radiant temperature equal to air dry bulb, 50-percent relative humidity, 30-fpm (0.15 m/s) air speed, 0.6 clothing insulation (Clo)
Heat Loss 2: 80°F (25.7°C) air dry bulb, mean radiant temperature equal to air dry bulb, 50-percent relative humidity, 120-fpm (0.51 m/s) air speed, 0.6 Clo

While the amount of heat loss from the body is the same for both scenarios, the body utilizes different modes of heat transfer to achieve thermal equilibrium/comfort.

FIGURE 1. Heat balance at 1.1 Met and various indoor-air conditions.

Under more extreme environmental conditions, the body may not be able to maintain a balance between heat generation and heat loss. With 100-percent relative humidity and still air, body temperature may begin to rise at an air temperature as low as 94°F (34°C).³ With low relative humidity and the movement of air, body temperature can be maintained at air temperatures above 125°F (52°C). The World Health Organization (W.H.O.) suggests air movement is beneficial only when air temperature is below 104°F (40°C).⁵ This, however, neglects the impact of humidity, which is accounted for in the Center for the Built Environment (CBE) Thermal Comfort Tool.

Resilience

As global temperatures rise and extreme-weather events become more severe and more frequent, the need for resilient buildings perhaps never has been greater. In the case of extreme-heat events, HVAC is the most common solution. HVAC, however, can be expensive to install, operate, and maintain and overwhelm increasingly taxed power grids. The subsequent sections will discuss how the risk of heat-stress-related illness or death can be mitigated, downtime reduced, and disruptions to business and everyday living limited.

Redundancy

Equipment redundancy—the running of multiple machines in parallel to ensure availability of service—is rooted in an unassailable truth: Things break and often at the worst time. For example, the HVAC system serving the main terminal at a regional airport was lost for a number of days one recent particularly hot summer. In the absence of air-circulating fans, there was no relief from the extreme heat: Workers were sweating and uncomfortable, while passengers awaiting flights were lying on concrete floors to cool off.

Consuming less energy and able to run on partial backup power or solar-generated power, air-circulating fans working in concert with HVAC can help to maintain continuity, not to mention help to preserve the reputation of a business, under even the most extreme weather conditions.

Table 1 shows CBE Thermal Comfort Tool calculations for a building under the following scenarios:

full HVAC system
no means of cooling occupants
air-circulating fans only
hybrid system of HVAC and air-circulating fans

The HVAC and hybrid systems provide thermal comfort within ANSI/ASHRAE Standard 55, Thermal Environmental Conditions for Human Occupancy, parameters (predicted mean vote [PMV] between +0.5 and -0.5). The scenario of no functional HVAC and no fans is high-heat-risk, with a predicted percentage of dissatisfied (PPD) of 99 percent. Comfort is improved in the air-circulating-fans-only scenario, as less than half of the occupants are dissatisfied and there is a dramatic reduction in heat risk.

TABLE 1. CBE Thermal Comfort Tool calculations for different building scenarios.

Refuge

In late June/early July 2024, Hurricane Beryl left more than 2.1 million Texans without electricity. In cities such as Houston, authorities established cooling and charging centers where people could escape the extreme heat. With public-health agencies such as the U.S. Centers for Disease Control and Prevention (CDC) and W.H.O. reinforcing the importance of these shelters as part of climate-change-adaptation strategies, organizations are exploring converting their spaces for this use. Given that power outages so often are the reason these centers are needed, efficient solutions are vital. Because of their low energy consumption, LDCF can operate on renewable energy with storage, providing life-saving comfort amid threatening conditions.

Table 2 shows design options for a 10,000-sq-ft, two-basketball-court gymnasium with a solar and battery-storage system in the suburbs of Washington, D.C., intended for use as an emergency shelter. Three limited-power scenarios are shown:

one of two 15-ton (53 kW) rooftop units (RTU)
one 18-ft- (5.5 m) diameter ceiling fan with four air changes per hour (ACH) of ventilation
four 14-ft- (4.3 m) diameter ceiling fans with four ACH of ventilation

Peak demand for the emergency comfort/lighting system (kilowatts [kW]), the estimated daily electricity use of the system (kilowatt-hours [kWh]), and the approximate size of the supporting solar array (direct-current [DC] kW) are shown for comparative purposes (Table 2).

Table 2 illustrates the extremely low energy use of the air-circulation-plus-ventilation design concept. As shown in the previous section, reasonable thermal comfort/low heat stress can be achieved, even at high air temperatures.

TABLE 2. Design options for a shelter-in-place gymnasium.

Regulations

On July 2, 2024, the U.S. Occupational Safety and Health Administration (OSHA) issued a proposed rule on the prevention of heat injury and illness in outdoor and indoor work settings. The rule would apply to all employers and be triggered when employees are exposed to temperatures of 80°F (27°C) for more than 15 minutes of a 60-minute period. Later in July, a similar regulation starting at 82°F (28°C) went into effect in California (California Code of Regulations Title 8, Industrial Relations).

Section 3396 of California Code of Regulations Title 8 applies to indoor work areas where the temperature equals or exceeds 82°F (28°C). Exceptions include remote locations of an employee’s choosing, some emergency operations, and some areas with short-term heat exposure. California is one of the first states with a heat-injury-related regulation. Expect similarly minded states to follow suit.

Table 3 provides an overview of the California regulation.

Conclusion

The data doesn’t lie: Extreme heat is becoming increasingly common, posing a threat to human health and life. As regulatory bodies strive to address risks and challenges from a legal perspective, there are simple changes that can be made to existing construction and elegant solutions that can be designed into new builds that will safeguard occupants and equipment while delivering productivity increases, reduced downtime, energy efficiency, and cost savings.

TABLE 3. Summary of heat-injury-related provisions of California Code of Regulations Title 8.

References

Kaplan, S. (2024, July 27). 4 hottest days ever observed raise fears of a planet nearing ‘tipping points.’ The Washington Post. Retrieved September 6, 2024, from https://bit.ly/Kaplan_0727
Howard, J.T., Androne, N., Alcover, K.C., & Santos-Lozada, A.R. (2024, August 26). Trends of heat-related deaths in the US, 1999-2023. JAMA.
Djongyang, N., Tchinda, R., & Njomo, D. (2010, December). Thermal comfort: A review paper. Renewable and Sustainable Energy Reviews, 14 (9), 2626-2640.
Heikens, M.J., et al. (2011, May). Core body temperature in obesity. The American Journal of Clinical Nutrition, 93 (5), 963-967.
World Health Organization. (2024, May 28). Heat and health. Retrieved from https://bit.ly/Heat_Health
California Department of Industrial Relations. (2024, August). Cal/OSHA heat illness prevention guidance and resources. Retrieved from https://bit.ly/Cal_OSHA_Heat

About the Author

As principal engineer, codes and standards, for Big Ass Fans, Christian Taber, BEMP, HBDP, CEM, is active in the development of building codes, standards, and regulations, which finds him collaborating frequently with Air Movement and Control Association (AMCA) International, ASHRAE, the International Code Council, and the U.S. Department of Energy. He is a member of the AMCA International board of directors.