“Sick Pool Syndrome” is a phrase that is relatively unfamiliar to the HVAC community. However, if there is a high school or college in your area with an indoor pool where swimmers train, there is a good chance that some of those swimmers suffer from Sick Pool Syndrome’s consequences: extreme shortness of breath or asthma events.

Correcting the problem holds opportunities for contractors interested in natatorium work. From the HVAC perspective, the problem is not just excessive humidity and corrosion from chlorine (although those considerations could be helpful in obtaining the necessary funding for the work).

The IAQ problem is caused by air stratification, which creates dead air at the water’s surface, where swimmers inhale what is essentially chlorinated air.


Swimming has been considered a healthy sports alternative for young people with asthma. In addition to easier breathing than higher-impact land sports, the deep breathing of swimming has even been considered beneficial to asthmatics. So it was not surprising that after a swim meet or during rigorous workouts, some members of the swim team would use their inhalers — there were just more asthmatics on the swim team, right?

According to Paul Richards, aquatics director at Dickinson College, Carlisle, PA, asthmatic members of his swim team routinely use their inhalers before swimming laps to prevent asthma events. The situation in Dickinson’s natatorium became alarming when those swimmers had to stop in the middle of exercising and use “rescue inhalers” because they couldn’t breathe.

Depending on how the pool water is cleaned, dead air at the water’s surface could be carrying chlorine gases. Richards explained that pool chlorine breaks down into hypochloric and hydrochloric acids and other compounds broadly called “chloramine,” which become trapped in that dead space 10 to 12 inches over the water’s surface. Swimmers inhale just above the water’s surface, where the chemicals are concentrated.

Even though the amount of chlorine measured at the water’s surface is relatively low, over the course of a workout, a swimmer breathes in much more of the chemicals, resulting in exercise-related asthma (bronchospasm), according to Drobnic, Freixa, Casan, Sanchis, and Guardino in their paper, “Exercising Increases the Toxicity of a ‘Safe’ Chlorinated Pool Atmosphere” (Medicine and Science in Sports and Exercise, 1996).

Still more alarming: In 2000, Dr. Stephen J. McGeady from Thomas Jefferson University, Wilmington, DE, measured the lung function of competitive swimmers in swimming pool and lab settings. He and his colleagues heard that many non-asthmatic university team swimmers had to use inhalers. This proved to be true. Richards said he was not surprised to hear it.

Richards said that ideally, air return intakes should be positioned to direct airflow so as to eliminate dead areas. However, he noted, “Ventilation [of natatoriums] was not a design issue for a long time.”

Dickinson College's poor natatorium IAQ was remedied with an overhead trunk fabric duct design (above) and a heat recovery dehumidifier (below).


Coach Richards joined Dickinson College in 1994. In addition to his coaching and teaching duties, Richards oversees the operations of Dickinson’s year-round aquatics facility. His background includes the study of natatorium mechanics.

There was no doubt in his mind that his natatorium suffered from Sick Pool Syndrome. In fact, he recounted that when he first arrived, the ventilation system was completely shut off because as he was told, “The chlorine odor was too strong in the parking lot.” He got that situation changed immediately, but the problems continued.

Swim practice was periodically interrupted by team members with breathing problems caused by stagnant, chloramine-laden air near the pool surface. While all of these team members were already known to have asthma, he agreed that “anyone with restricted airway disease” is susceptible to Sick Pool Syndrome. He was not surprised to hear of the reports of non-asthmatics suffering bronchospasms while swimming.

The 23-year-old Dickinson natatorium was built in the 1980s. It was originally designed with a commercial air conditioner. Its air distribution consisted of two 4- by 4-foot wall grille returns in one location, and a series of five 3- by 1-foot wall diffuser air supplies in only one wall. Consequently, air stratification was significant in more than 50% of the pool area. Air movement was particularly dead at the pool’s surface level, where heavy chloramine gases became trapped.

In addition, 80% humidity levels had taken a toll on many parts of the 10,000-square-foot natatorium structure, as well as the roof and metal amenities of Kline Athletic Center, a 78,000-square-foot field house.

Ongoing roof problems in the adjacent field house eventually led school officials to appropriate $248,000 for repairs. These included a complete retrofit for Dickinson’s eight-lane, 25-yard-long pool natatorium. Richards, who has a masters degree in sports sciences with a specialization in aquatics maintenance, management, and design, researched natatorium technology with the help of Durwin Ellerman, supervisor of mechanical and electrical trades at Dickinson.

Natatorium environments are most effective with a combination of under-deck and overhead air supplies, according to the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE).

However, this was not feasible with the school’s budget.

Rich Munkittrick, vice president of manufacturers representative H&H Sales, Mechanicsburg, PA, provided drafting and engineering assistance. Since under-deck ductwork was not economically feasible, Richards and Ellerman conceived of a main, 52-inch-diameter trunk line spanning 120 feet down the center of the natatorium. Their design incorporated a heat recovery dehumidifier from Dectron Internationale (Roswell, GA) and fabric duct from DuctSox (Dubuque, IA), which could improve the pool’s indoor air quality (IAQ) while staying within budget.


The trunk line delivers approximately 15% of the airflow through the fabric’s natural porosity, according to its manufacturer. The remaining air is delivered through a linear diffuser and four 20-inch-diameter, perpendicular branches that spray the windows and the spectator section with 82 degree F air.

Air distribution at the pool’s surface level, which was a major concern of Richards because of swimmers’ health issues, now relies on new returns at the shallow end to draw the conditioned air down from the supply duct to mix with evaporating chemicals and then return them to the dehumidifier.

For the best air distribution and aesthetics, Richards said he wanted the trunk line at the center of the roof’s peak; however, existing lighting fixtures would have been blocked by the duct.

Ellerman conceived of a fixture retrofit that would allow both the trunk and the lighting to hang at the center. He lowered the lighting below the trunk line by extending the ceiling-mounted conduit pendants into an “O” shape that encircles the duct and connects to the fixture below the duct.

Another important factor is the temperature differential between the air and water (which now measure 82 degrees and 80 degrees, respectively). Previously, the differential between the 75 degree air and the 80 degree water caused additional humidity problems. The current relative humidity is maintained at 50%, thanks to the improved air-to-water temperature differential and the addition of the heat recovery dehumidifier.

Richards said that the conditions now are ideal. And the dehumidifier recaptures condensate, “a complete pool fill per year,” said Richards.

“I guess the jury is still out,” on how well the fabric ducts will stand up to the corrosive environment, he said. “I think it will last a lot longer than metal.” He also pointed out that it was “very easy to install,” hence, very easy to replace. And it can be taken down by hand and washed.


The most important consideration, of course, is the swimmers’ health. These days, inhaler use is minimal at practices, according to Richards.

Visiting spectators and opposing teams regularly make positive comments on humidity levels and IAQ during swim meets held at Dickinson, he said. “We did a survey in our regional conference. Based on chemical evidence and anecdotally, we were certainly not the exception” for Sick Pool Syndrome.

He suspects a significant number of natatoriums nationwide may need complete retrofits or serious fine-tuning to their air-handling and chemical systems, as well as their operating procedures.

However, knowledge of poor IAQ in natatoriums may be lacking among high school and college administrators. Even in his own school’s case, it was the roof deterioration that finally got funding approval for the field house.

HVAC contractors may need to educate aquatics directors on humidity control, air stratification, and the role they play in Sick Pool Syndrome.

“Colleges and universities tend to hire swim coaches and then name them aquatics directors,” Richards said. “Unfortunately, a large percentage of swim coaches have little or no training in facilities management and know very little about water chemistry. So then those responsibilities are left to physical plant engineers, who many times don’t have aquatics facility training either.”

Sidebar: It All Starts With Good Design

When mechanical contractors design an enclosed swimming pool, they have to keep in mind how to correctly distribute the air throughout the space and how to remove air from the room efficiently, in order to avoid air stagnation or stratification in the natatorium.

Controlling humidity in a natatorium presents many challenges. Special attention and careful consideration must be given to the location of supply air ducts, the location of the air return grille, the use of moisture barriers, and door and window insulation values, according to Pat Reynolds, president of PoolPak Inc.

Obviously, a well-designed dehumidifier is only one step toward effective climate control in a pool space.

According to Reynolds, “Efficient dehumidification of a pool enclosure requires well-balanced and properly placed ducting systems. Ducts should never be positioned in a manner such as would result in the short cycling of the supply air. Short cycling is caused when the location of the return duct is too close to, or directly in line with, the supply duct causing warm, dry air to recycle prematurely.”

The return intake(s) should be positioned so that all of the moist, warm air flows efficiently back to the dehumidification system, eliminating dead areas where air stagnation can occur, Reynolds stated. “In most instances, a single return duct is ideal in the pool area. The desirable location for the return is at a point high enough to capture the warm, humid air that naturally rises.” Normally, this is about 10 to 15 feet above the floor, or the surface of the pool.

“Airflow should never be directed over a pool surface or over any concentration of water,” Reynolds said. “Should air flow over or too close to the water, it will speed evaporation and limit the effectiveness of the dehumidification system. The greater the velocity of the air currents, the greater the evaporation process.”

Typically, an indoor pool requires space air heating 70% to 90% of the year. Therefore, the most effective air distribution system is one that takes advantage of hot air’s natural tendency to rise. This type of system will supply the air “low” and return it “high,” Reynolds pointed out. “When this is not possible, a ceiling supply arrangement is necessary.”

The supply air grille should be located close to the windows, preferable within 12 inches from the surface to sufficiently bathe the cold glass with a blanket of warm, dry air. The majority of the supply air (80%) should be directed down the walls. The remaining 20% should be directed along the ceiling to break up any stratification and stagnation that might occur there.

Where skylights are present, it is best to utilize supply ductwork to flood the glass with warm, dry air. Another method to deal with skylights is to install ceiling fans, running in reverse, to draw up the warm air against the glass surface. This, however, is not as reliable as a direct flow of air from ducts.

Reynolds points out another important factor: “You can correctly build the enclosure and have a very good dehumidification system, but if you do not maintain your equipment, you cannot control the humidification adequately.”

Publication date: 08/12/2002