When it gets too hot indoors, worker performance levels decrease.

Feeling the Heat

The ideal temperature for indoor work is to some degree subjective, but the range for what is acceptable is fairly narrow.

As adaptive as we humans are as a species, our capacity to work--or even think straight--for sustained periods is contingent on conditions being relatively right or our having special gear or apparel to help us when they are not. Human performance is affected by all kinds of factors, from the environmental and emotional to the physical and pharmaceutical, but studies show we are especially susceptible to even mild fluctuations in the ambient temperature under which we toil.

Thermal comfort is, of course, the main reason we have buildings. The ideal conditions inside those buildings are essentially subjective and depend on such variables as the season, geographic region, a person's metabolic rate or activity level, and the amount of insulation gained from clothing. But managers seeking to provide the optimal indoor working conditions for the greatest number of people should consider their facility's temperature and understand how airflow and humidity affect it, as well as what effects the thermal conditions are likely to have on employee productivity.

OSHA has no regulations specifically addressing indoor temperature and humidity, having withdrawn its Indoor Air Quality proposal and terminated its rulemaking proceeding back in 2001. But in Section III, Chapter 2, Subsection V of the OSHA Technical Manual, the agency does recommend temperature control in the range of 68-76 degrees Fahrenheit and humidity control in the range of 20 percent to 60 percent. Otherwise, the agency defers to the American Society of Heating, Refrigerating, and Air-Conditioning Engineers' Standard 55-2004, Thermal Environmental Conditions for Human Occupancy, as a source of guidance.

Standard 55 discusses thermal comfort within the context of air temperature, humidity, and air movement and provides recommended ranges for temperature and humidity that are intended to satisfy the majority of a building's occupants with activity levels typical of office work, but the standard does not cover hot or cold stress in thermally extreme environments or comfort in outdoor spaces. Instead, it provides a range of operative temperature limits similar to those OSHA recommends, but it does so assuming that the conditions it specifies will have an overall acceptability rate of only 80 percent, leaving 20 percent of workers experiencing either complete or partial local thermal discomfort.

The Human Factors Design Handbook, 2nd Edition, a standard reference for facility designers, gets more specific. It says the preferred temperature for year-round sedentary activity while wearing light clothing in an indoor facility with relatively still air and normal (40 to 60 percent) humidity, such as found in a typical office environment, is 72 degrees F. If the airflow is increased by opening windows or using ceiling fans (or even fog fans, in warehouses, say), temperatures up to 76 degrees F are acceptable and, depending on the work setting and activity level, possibly even preferred. The handbook adds, though, that without that additional airflow, anything much higher than 74 degrees F starts becoming a problem. Dr. David Curry, vice president of Human Performance Sciences & Technology at ITC Experts in Sugar Grove, Ill., agrees with those numbers.

"You get to 75 [degrees F] under normal indoor conditions, with limited airflow and normal humidity, you're going to start seeing a decrement in performance," Curry says. "I'm not saying workers won't be functional, but they're going to get tired faster, show a little bit more physical fatigue and lethargy, and their work output is going to drop. You get to 78, you get a fairly pronounced decrement in terms of both mental and physical output. In other words, psychomotor performance suffers. People start falling asleep. That's about the optimal temperature if you're taking a shower or bath, but then, obviously, you're naked."

Hits of the Eighties
If the indoor temperature reaches 80 degrees F and the airflow and humidity level are normal, meaning fans are not moving the air around and no breeze is to be had through an open window, nothing is optimal for workers. The HFD Handbook describes that temperature as the "[m]aximum for acceptable performance even of limited work" and says the heat at that point reduces work output "as much as 40-50 percent" and creates nasal dryness in most people. Curry notes worker safety is already a factor at 80, and the performance metrics continue deteriorating as the temperature rises.

"You're going to be getting complaints from everybody when temperatures are in the 80- to 90-degree range," he says. "At that point, the psychomotor performance decrements and physical fatigue are kicking in in a serious way."

The HFD Handbook describes 90 degrees F as the "[u]pper limit for continued occupancy over any reasonable period of time." The implication of "reasonable" in that sentence is "not very long," depending on the all-important factors of airflow and humidity. Indoors, increasing the level of air movement is beneficial for improving comfort at higher temperatures in the same way even a slight outdoor wind can make conditions more tolerable in high heat. "It's the same effect you get out of wind chill, although we don't call it that when the temperature is around 85 degrees," Curry says. "To create that indoors, though, what you're looking at are ventilation requirements in order to mitigate the heat stress."

Air Apparent
Curry says the optimal amount of air movement in an indoor facility ranges anywhere from 5 to 50 cubic feet per minute, depending on whether the weather is hot or cold, the size of the space, and the number of workers involved. "Obviously, if you cram 50 people into a room, you start getting the bodies actually radiating heat at normal body temperature," he says. "If you go into a conference room on a warm day by yourself, 72 degrees is fine in there, but if you have 50 people in that same room, it starts getting progressively warmer because then you've effectively got 50 little heaters running around. For most tasks, though, an average of 25 cubic feet per minute is what you want."

ASHRAE's Standard 55 addresses the relationship between increased air speed and improved comfort using calculations of equivalent heat loss from the skin combined with professional judgment about reasonable limitations that should be placed on this allowance. The standard does not, however, specify a minimum humidity level because no lower humidity limits relate exclusively to thermal comfort. Curry notes, however, that the humidity level often has everything to do with thermal comfort. Anecdotally, he recalls his own experiences in two drastically different environments.

"I used to live in Phoenix, and I was perfectly comfortable at 100 degrees if I wasn't in direct sunlight," he says. "On the other hand, I grew up on the very southern coast of Alabama, and there it was a race every day to see whether the temperature or the humidity index hit 100 first, and it was absolutely miserable. It makes a big difference as far as the humidity level is concerned."

OSHA provides guidelines and addresses complaints about IAQ related to temperature, humidity, and lack of outside air ventilation at its Web site, www.osha.gov, but it notes there that matters of human comfort do not fall into its bailiwick. "Not until workplace IAQ problems move beyond matters of human comfort into potential hazardous conditions leading to serious physical harm or death, do OSHA standards become applicable," it says. With that caveat, the agency further notes that the types of indoor workplaces the agency is more prone to focus on regarding IAQ include foundries, boiler rooms, bakeries, commercial kitchens, laundries, and the like--not office buildings, per se. For those, says Curry, "You generally want to keep the temperature at about 72 to 74. That's sort of the optimal range."

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