Indoor Air Quality and Evaporative Cooling
IAQ and Health
Indoor Air Quality (IAQ) affects our health, well being, comfort, and productivity. A large majority of us spend our work hours indoors in offices, factories, or other industrial facilities. The lion's share of the remainder of our time is spent inside our homes.
Hazards that can affect indoor air quality and therefore our health can be in the form of chemical hazards; biological hazards; and physical hazards.
The following conditions define healthy indoor air quality:
A 1992 study by Armstrong Laboratory at Brooks Air Force Base in Texas identified the three most frequent causes of unacceptable indoor air quality:
These continue to be most frequently pointed out as the cause for IAQ issues.
With poor IAQ, our health and productivity suffers. An indicator of this from a health perspective is the number of sick days used by the work force. Measurements of productivity impact are basically quality and quantity of results achieved be it hardware parts produced, drawings completed, calculations completed, letters produced, calls answered, and a myriad of other outputs.
Studies conducted by the U.S. Environmental Protection Agency (EPA) and others show that indoor environments frequently have levels of pollutants sufficient to classify as health risks. Most of the health affects are not affected by the humidity of the air directly but by standing water and condensed moisture on surfaces within the facility. Dust mites, which are found in carpet, bedding, and furnishings, are influenced by the moisture content of the surrounding air.
In 1966, ASHRAE Standard 55-1966, Thermal Environmental Conditions for Human Occupancy defined the most widely accepted measure of thermal comfort: "Thermal comfort is that condition of mind that expresses satisfaction with the thermal environment." Therefore, thermal comfort is a judgment people make using their sensory input from physical, physiological, and psychological experience of their environment. People determine their state of comfort from direct temperature and moisture sensations from the skin, deep body temperatures, and the response of the body to regulate its temperature.
A typical person is comfortable when body temperatures are held in a tight range, skin moisture is low, and the physiological process to regulate these temperatures is only moderately challenged. Our bodies' comfortable core temperature is about 98.2 F at rest, 99.3 F when walking, and 100.2 when jogging. Comfortable skin temperatures at rest are between 91.5 and 93 F, and increase with activity.
The body regulates temperature by transferring sensible heat by conduction and convection between the skin and the surrounding air and by latent heat through respiration and evaporation of sweat. An average person at rest in an environment at 75 F and 50% relative humidity will transfer 20% of its energy via evaporation and 80% via dry or sensible heat transfer mechanisms. Decreasing the relative humidity from 50% to 20% increases the heat transferred via evaporation from 20 to 25%, which lowers the skin temperature about 0.5 F. Under these conditions, the air temperature would have to raise 1.8 F to maintain the same skin temperature and feeling of warmth.
These conditions of comfort were identified in a 1995 study of the temperature and relative humidity that occupants at rest would identify as comfortable and are shown in the table below:
IAQ and Productivity
I think you will agree that experience tells us that mental capacity, concentration, and motivation suffer as temperature and humidity get out of our comfort zone. We have all experienced the challenge of attempting to stay alert in meetings when the room temperature is a little warm. While this correlation seems clear, the complexity of setting up tests to measure and assess such causal impacts has prevented such undertakings, which limits the available data.
Researchers set about looking for correlations of productivity and indoor air quality beginning with the Hawthorne Works studies of Western Electric in the 1920s. These studies are famous not for identifying such correlations but for finding that in itself involving the subjects was sufficient motivation to improve their productivity. Early studies of the effects of climate on human performance confirm what common sense and reason would predict:
In 1968 NASA found and reported in a heat stress report CR-1205 (1) that temperatures over 75 F have a marked NEGATIVE effect on both the productivity and accuracy of work. This study and its results are still the authoritative reference used as the foundation for NASA standards and design. The following table is a summary of the relationships identified during NASA tests between temperature, work output and accuracy.
A Swedish study of secondary schools looked at the subjective perception of impaired mental performance due to poor IAQ . The study measured subjective perception by asking participants for their judgment or perceptions of various changes in IAQ on their mental performance. Their responses were sufficiently consistent to demonstrate a statistically valid relationship between impaired mental performance, measured indoor air pollutants, and low makeup airflow rate. In three independent investigations, Schweisheimer (1966) reported significant productivity improvement by workers with the installation of air conditioning:
IAQ and Evaporative Cooling
Today's indoor air quality is largely influenced by actions taken over the course of the past several years by industry, regulators, and owners to conserve energy resources and minimize costs. In particular, the energy crisis of the 1970's raised the level of awareness on energy supplies and consumption associated with heating and air conditioning of homes and buildings. Mechanical air conditioning of buildings is a large energy user with the air in the building being cooled by recirculating it through cooling coils that have been cooled by a refrigerant. To improve the energy efficiency of buildings being cooled in this manner, actions were taken to insulate and isolate the conditioned air space with the objective of reducing the sun heat load and the heat load from outside air intrusion. In this approach, the most energy efficient strategy is to have a very low makeup air to avoid the large heating and cooling load associated with conditioning this makeup air. In tension with this approach of restricting the makeup air volume to save energy is the need or value associated with providing fresh air and exhausting air with raised levels of carbon monoxide and carbon dioxide and other contaminants.
Evaporative cooling on the other hand cools buildings by bringing in outside air and passing it through the evaporative section of the cooler at a rate that is sufficient to move and distribute cool fresh air throughout the building and purge all areas with local heat loads such that this heat is carried out of the building. The rate of outside air flow into the building to achieve this cooling is typically several times that required to satisfy any regulatory or code minimum flow requirement and is adequate to assure that a healthy environment is achieved and maintained.
Direct evaporative cooling is limited by the ambient wet bulb temperature and indirect evaporative cooling is limited by the ambient dew point temperature. As temperatures approach these limits the relative humidity of the cooled air approaches 100 percent or saturation. A New York State Commission Study found the level of productivity to be unaffected at 80 percent humidity levels and temperatures less than 80 F.