Improving Indoor Air Quality

In December 2005, 14 people were killed and 11 more were injured in a Russian indoor swimming pool when a concrete ceiling, supported by stainless steel rods, collapsed (Associated Press, 2005). The conditions that led to this incident and other similar aquatic facility catastrophes can be traced to stress-corrosion cracking that develops due to poor air-quality management within the facility (Dumas, 2006). In natatoriums with “modern,” energy-efficient air handlers, recirculation of conditioned air makes it difficult to rid the building of harmful chloride-ions and chloramines that can cause stress-corrosion cracking to occur.

Facility managers and pool operators must learn to understand the delicate relationship between the active water chemistry, the influence it has on air quality and the distribution of potentially bad air through the ventilation system. Improper management can contribute to catastrophic structural failure, untimely structural rebuilding and human sickness, while proper management will contribute to a healthy and safe environment.

Defining The Problem

Indoor pool facilities, or natatoriums, differ in design, construction and maintenance requirements from all other recreational structures (Williams, 2006). Natatoriums have three interacting components--the water, the indoor air, and the ventilation system. Balancing these interdependent components makes natatoriums difficult to manage. Consequently, it is imperative that aquatic facility managers understand the interaction of these elements inside a natatorium, including relative humidity, ambient air temperature and water temperature, all of which contribute to overall air quality, and how the air circulation system affects air quality.

As chlorine disinfects and reacts with organic compounds, like dirt, urea and body oils, typically introduced by swimmers, it produces natural byproducts. The primary byproducts produced in a gaseous form are commonly called chloramines. When this occurs at outdoor pools, the chloramines rise into the atmosphere and are carried away by air movement. However, when this occurs inside a natatorium, as shown in Figure 1, the chloramines are trapped inside the building until the ventilation system can exhaust them, A buildup of chloramines causes the offensive “pool smell” that not only is harmful to humans, but also can compromise the structural integrity of the building. Current energy-saving practices, like recycling of the air, promote high levels of chloramine buildup, decrease the breathable air quality, and increase contact with structural steel. In addition, when high levels of humidity are allowed to condensate, the impact of chloramines greatly increases the potential for corrosion.

Humidity Is Relative

In the natatorium, humidity control is one of the most important objectives (Williams, 2006). The operator must be aware of and minimize excessive moisture buildup, as it can increase health risks and condensing on surfaces, causing devastating effects on the infrastructure and fixtures. Condensation is an aggressive form of water and contributes to both concrete and metal degradation, which can eventually compromise the building’s integrity. If any chlorine compounds, like chloramines, are in the air, these molecules will find their way into the moisture and significantly enhance the damage created by condensation.

In a typical natatorium, relative humidity should be maintained around 50 percent (Osinski, 2006) in order to prevent increased health risks associated with humidity levels below 40 percent and above 60 percent. The target level is complicated because swimming pools continue to generate moisture, even when people go home for the night. It takes less than 30 minutes for the relative humidity of a natatorium to increase to well over 80 percent if the ventilation and dehumidification system shut off (Xie and Copper, 2006). Managers and operators must understand one can never turn off the air-recirculation system.

Temperature Control

Proper air temperatures also must be maintained in the natatorium year-round--through heating or cooling--for guest comfort and minimizing pool-water evaporation. Although the water probably feels hot to those in street clothes, most experts agree that the ambient air temperature of indoor pool environments should be maintained 2 degrees Fahrenheit above water temperature. Therefore, a competitive venue with a water temperature of 80 degrees should maintain the air temperature around 82 degrees. Proper temperature management will prevent excessive humidity levels and unnecessary pool water heating since the evaporation rate actually increases as the air temperature decreases.

As therapy and leisure pools continue to be popular with designers and communities, the environment can become increasingly miserable for staff, such as lifeguards, as air temperatures rise to 86 degrees and higher. With studies showing that appreciable decreases in vigilance occur at temperatures above 84 degrees (Griffiths, 2003), supervisors must require lifeguards to use proven methods to maintain a high level of vigilance during swimmer surveillance.

Indoor air temperature and relative humidity used to be maintained by bringing in large amounts of outside air, heating it, blowing it across the pool and exhausting equal amounts of humid and chemical-laden air out the other side. Today, energy conservation and total environmental control are primary concerns of many agencies. Indoor swimming pool environments require a large amount of energy for heating, cooling and dehumidification. With the continually rising costs of fuel, conventional methods of operation have become economically impractical, inspiring new methods.

Newer systems seem to have learned a lesson from the residential market. At home, when people want to control the environment precisely, they close doors and windows and turn on the air conditioner to maintain the temperature and recirculate the air. The same principle has been applied to natatoriums. However, due to higher temperature and humidity, the load on equipment is much different than normal air-conditioning applications. Also, the nature of the chemical processes in the pool water do not allow for the space to be treated similarly.

The Drawbacks

Understanding the relationship between water chemistry and air quality is crucial. As chlorine sanitizes in a pool, heavier-than-air disinfection byproducts are released into the surrounding atmosphere. Unless circulating air carries the gasses out of the building, they will linger over the pool. When people complain of that offensive chlorine odor, what they’re really breathing are noxious ammonia compounds of chlorine, commonly called combined chlorine or chloramines. In addition to the “pool smell,” chloramines are also the dominant contributor to swimmers’ eyeburn (Williams, 2006).

Many public water-supply companies now are using chloramines--usually bound to ammonia--to disinfect the drinking water. When this chloraminated water is added to pools to make up for splashing, evaporation or backwash, the combined chlorine level in the pool water inevitably rises. In this situation, it is nearly impossible to superchlorinate enough to reduce the chloramine level to an acceptable range, and the impact on indoor air quality will be even more severe.

In the attempt to be energy-efficient, a new air quality problem has developed. As chloramines pollute the air, this bad air is being recirculated with minimal fresh-air exchange in order to conserve heating and cooling costs. This problem didn’t exist in natatoriums 40 years ago because of the large amount of fresh air constantly introduced, but the air-handling systems were considered wasteful. Some conditions that have changed in recent years include:

· Generally higher operating temperatures, leading to increased chemical activity

· Greater usage and aerosolating water features, resulting in more dispersal of pool water into the atmosphere

· Recirculation of both pool waters and atmospheres to conserve fuel, leading to higher dissolved solids and contaminants in the air

Adoption of ceilings suspended by stainless steel wire and rods to improve acoustics.

These conditions are frequently associated with stress-corrosion cracking failures (Oldfield and Todd, 1990). They lead to an increase of aggressive chemical-laden moisture in the surrounding atmosphere, and provide a focus for the attack. Stress-corrosion cracking seems to be a product of exposure to moist atmospheres that are high in chloride levels. This type of corrosion is never found around outdoor pools or even underwater (Oldfield and Todd, 1990).

Natatorium air-quality issues also have attracted the attention of the Centers for Disease Control and Prevention, which recognizes breathing air loaded with chloramines can lead to symptoms ranging from mild wheezing to lung disease and asthma (Centers for Disease Control and Prevention, 2007). Chlorine byproducts have long been known to trigger asthma attacks. Current research suggests that prolonged exposure can damage the lungs, making people more susceptible to many respiratory illnesses, including asthma, hay fever, sinus inflammation and chronic cold (Yu, 2007a and Yu, 2007b).

Poor indoor air quality can only be resolved at the underlying cause. It requires good water-chemistry management and proper ventilation of the natatorium space. This involves a combination of efforts involving everyone from the building architect and ventilation engineer to the facility manager and pool operator. The solution is a two-fold approach that requires good air circulation supplemented with adequate outside fresh air and maintenance of breakpoint chlorination to oxidize contaminates out of the water.

Improve Air Quality

A primary component of improving the air quality of indoor swimming pools is ensuring there is an adequate amount of ventilation to remove chloramines from the environment. Many facilities fall victim to only complying with minimum requirements, such as ASHRAE Standard 62, Ventilation for Acceptable Indoor Air Quality. The American Society of Heating, Refrigerating and Air-Conditioning Engineers is currently conducting research to determine if this is applicable to natatoriums, where conditions are harsher than residential space (American Society of Heating, Refrigerating and Air-Conditioning Engineers, 2007). At least eight complete air exchanges per hour should be made, with 10 exchanges for busier pools, or facilities with water features. The air being turned over should also be constantly supplied with a minimum of 40 percent outside air with the ability to augment that to 100 percent outside air during peak hours and superchlorination (Osinski, 2006).

The design of the air-distribution system is perhaps more important than the air-exchange rate (Xie and Cooper, 2006). It is important to ensure air goes where it is needed to prevent aggressive condensation and pockets of poor air quality, which can have disastrous consequences. A common design mistake is to locate all supply- and return-air locations near the ceiling. This does not enable the air to reach the pool and deck areas. Another complication of this mistake is it short-circuits the system. Supply air should be introduced low in the natatorium, with sufficient velocity to blanket the coldest surfaces and uniformly diffuse air across the surface of the water. Although this airflow increases the rate of evaporation, it is necessary to move the disinfection byproducts away from the pool surface and out of the natatorium. Returns should be strategically located at the ceiling level of an interior surface for a cross-ventilation effect. Returns must also be adequately sized to prevent pockets of dead air and reduce noise. An auxiliary exhaust fan can help control humidity, especially near a spa or spraying water feature.

The other component to improve air quality is proper water-chemistry management practices. Despite popular belief, the major cause of the production of chloramines is too little “free” chlorine for too long. A procedure of maintaining breakpoint chlorination will avoid excessive chloramine production that pollutes the air. This involves conscious intervention in pools where chloramines build over time. The pool must be superchlorinated to rid it of the irritant, and then a higher target residual of chlorine is consistently maintained. Many pools need 3.0 to 5.0 parts per million (see Table 1) to maintain breakpoint and prevent chloramine formation.

When combined chlorine levels do reach an intolerant point for swimmers and staff, the pool must be superchlorinated to a calculated value. This procedure may be more appropriately called breakpoint chlorination but never “shock.” The commonly accepted threshold for combined chlorine is 0.4 parts per million, but this is subjective (Williams, 2006). Unfortunately, classic superchlorination procedures often fail when applied indoors. Although there are many possible reasons, usually the oxidized materials are not moved out of the building and then become re-saturated in the water. When performing superchlorination indoors, it is absolutely essential to increase the air-handling system to bring in 100 percent fresh air or operate it in “purge” mode, along with opening all doors and windows. It also helps to get fans blowing chloramine-filled air across the pool and out the other end.

Alternative Solutions

While the answer to the stress-corrosion cracking and health-risk issues of indoor pool air containing chlorine compounds is in the proper ventilation of the space and sound management of water chemistry, other options do exist to help alleviate the problem. Supplementary systems, such as medium-pressure ultraviolet light and corona discharge ozone, have been applied successfully. The use of non-chlorine oxidizers, like monopersulfates or a new class of chemicals called peroxolytes, along with regular chlorination, has shown promising results (Martin and Williams, 2007). Replacing the silica sand in filters with zeolite will help retain ammonia from being reintroduced to the pool, but this doesn’t help once the ammonia has combined with chlorine. Side-stream, granular-activated carbon filters also lower chloramine levels, and remove other chlorine compounds. This can be a useful addition to the fresh water supply for facilities combating chloraminated source water. Educating guests about their role in indoor-pool air quality can eliminate part of the problem. Strictly enforcing showers before entering the pool may help. After all, it is body oils and sweat that combine with chlorine to produce the chloramines. Draining part of the pool and replacing it is a common practice in Europe, where the DIN standard requires replacement of at least 8 gallons per day. This dilution act is beneficial as long as the fresh water is not contributing to the problem.

Make It Work

Understanding the interaction of the natatorium’s environmental components, including air humidity, temperature, water chemistry and the ventilation system, will allow aquatic facility managers and operators to successfully maintain a healthy environment and corrosion-free building. Although the pressure to lower operating costs has led to many facilities recycling corrosive air, clearing natatoriums of chronic chloramines just takes proper ventilation and proactive management of water chemistry. Remember, facilities with the highest air quality are traditionally the worst in terms of energy conservation, but high energy bills may be a better alternative than structural failure and long-term health impacts.

strong>Joseph T. Walker Ph.D. is an assistant professor of Recreation at the University of North Texas. His recreation background includes aquatics, community/special event programming, facility operations/development, staff management and comprehensive planning.

Matthew D. Griffith is a graduate student at the University of North Texas in Recreation and Leisure Studies. He has an extensive aquatics background managing municipal, private, school district and university programs. As an instructor, Griffith has taught CPO courses around the country, as well as holding numerous other certifications in pool operations and service.


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Dumas, B. (2006, January 9). Russian pool roof collapse kills 14. Pool and Spa News.

Griffiths, T. (2003). The complete swimming pool reference (2nd ed.). Champaign, Illinois: Sagamore Publishing.

Martin, R., & Williams, K. G. (2007). What th’ heck are peroxolytes. Pumproom Press, 36, 1-3.

Oldfield, J. W. & Todd, B. (1990). Ambient-temperature stress-corrosion cracking of austenitic stainless steel in swimming pools. Materials Performance, 29, 57-58.

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Table 1

Natatorium Air and Water-Chemistry Parameters

Relative humidity: 40 – 60%

Air temperature: 2˚F above pool water temperature

Air exchange rate: 8 – 10 exchanges per hour

Outside air introduction: 40 – 100% depending on usage

Air distribution: Locate air supplies low to blanket cold surfaces, and move air across pool surface.

Locate adequately sized returns near ceiling level of an interior wall.

Pressurization: Natatorium should be positive pressure as compared to the outdoors, and negative pressure as compared to surrounding indoor areas.

Oxidation reduction potential (ORP): 750 – 850 mV

“Free” chlorine: 3.0 – 5.0 ppm as needed to maintain desired ORP

Combined Chlorine: Less than 0.4 ppm

pH: 7.0 – 7.8