The old days of walking into a sports and recreation facility and being hit with the strong smell that indicates the presence of a swimming pool are long gone. As the aquatics industry has grown so has the public’s expectation of a healthy, safe and inviting environment for indoor public swimming. Modern natatoriums (indoor swimming pools) are designed to ensure health and comfort for bathers, spectators, facility staff and the protection of the building and components. The most difficult of these conditions to regulate is the air quality.
Recipe for Good Air Quality
The recipe for good air quality is a result of facility and system design, operations and programs in this space. The proper control of water and air temperatures, air relative humidity, air ventilation and pool water chemistry balance will all contribute to the air quality in the natatorium. To guide us on these issues, we can look to the following standards and guidelines:
- The Model Aquatic Health Code Section 4.2.2
- ASHRAE Standard 62.1, Ventilation for Acceptable Indoor Air Quality
- ASHRAE Standard 90.1, Building Envelope Design
- Aquatic Facility Operator Manual, Chapter 20 “Concepts of Automation and Control,” by the National Recreation and Park Association
ASHRAE Standard 62.1 classifies natatorium air as class 2 air: Air with moderate contaminant concentration, mild sensory-irritation intensity, or mildly offensive odors. Class 2 air also includes air that is not necessarily harmful or objectionable but that is inappropriate for transfer or recirculation to spaces used for different purposes. ASHRAE 62.1 requires a minimum of 0.48 cubic feet per minute per square foot of pool water surface and pool deck, plus 7.5 cubic feet per minute per spectator in the natatorium.
In addition to this outside air, ASHRAE 90.1 requires a minimum of four to six air changes per hour for a natatorium and six to eight air changes per hour for the spectator areas: Both are the minimum standard requirements. For pools with greater loads, such as therapy pools or indoor waterpark features, the facility should be designed and operated at the upper limits or beyond to assist with good air quality. In the absence of these increased air changes or ventilation, the air may become class 3 air which is classified as: Air with significant contaminant concentration, significant sensory-irritation intensity, or offensive odor.
Humidity and Air Temperature
Patrons are typically more sensitive to humidity than they are air temperature, so trying to achieve a balance of the relative humidity and the natatorium air temperature can be key. To minimize the amount of water evaporation and overall loads to humidity, maintaining air temperatures 1-2 degrees warmer than the pool water temperature is ideal. When the water temperature goes up for pools, such as for instructional and therapeutic pools, the higher air temperature is not practical for bather comfort. The table below gives examples of the conditions for typical types of swimming pools.
Conditions for Typical Types of Swimming Pools
Type of Pool Air Temp ⁰F Relative Humidity % Water Temp ⁰F Competition 75 to 85 50 to 60 78 to 80 Recreation 80 to 85 50 to 60 82 to 86 Wading 78 to 85 50 to 60 84 to 90 Instructional 80 to 85 50 to 60 84 to 90 Therapy 80 to 85 50 to 60 86 to 94 Spa 80 to 85 50 to 60 100 to 104
Air movement across the pool water surface is another air quality factor. For bather comfort, it’s best to minimize air movement and any drafts at pool water level. However, for the elimination of chloramine fumes (the chlorine smell in air), greater air movement across the pool water allows chemical off-gassing, preventing foul-smelling air. The greater the air movement across the pool water, however, the higher the water evaporation rate. In general, try to have a maximum air velocity of 20-30 feet per minute to achieve off-gassing and minimize evaporation.
In addition to balancing air quality, the rates of off-gassing and evaporation must also be balanced with operating costs. For example, an increase in water temperature from 82 degrees to 84 degrees, will see an almost 10 percent increase in the HVAC energy costs. Conversely, reducing the HVAC system from six to four air changes per hour will see an almost 20 percent reduction in HVAC energy costs.
The most important aspect of the HVAC system to air quality is the ability to remove the air with the most contaminants from the natatorium. This air is heavier and, therefore, typically is located directly above the pool water. The location of both the supply air and return air HVAC openings to allow this removal to occur can reap huge benefits in air quality. For existing facilities that cannot adjust supply and return openings, large-volume, low-speed, overhead fans can be very helpful.
Water chemistry balance is the final ingredient in this recipe for good air quality. Maintaining constant break point chlorine levels and/or the use of ultra violet supplemental disinfection to prevent or minimize chloramine formation can contribute greatly to the water and air quality.
The right design and operation of your sports and recreation facility will help everyone to breathe a little easier.
Dennis Berkshire is President of the Aquatic Design Group.