Making Sense of Indoor Air Quality Metrics

Indoor air quality is not as simple as maintaining a HVAC system or fixing a leaking duct. It is constantly changing and is affected by a complex set of factors. Some of these factors can be external like pollutants or odors and some internal like poor ventilation systems and moisture and humidity levels.

Whatever is the cause of variation of Indoor Air Quality (IAQ), it is important for it to be controlled so that proper quality of air is maintained.

We assume that outdoor air quality is worse than indoor air quality. Recent research has proven that this is not true. Apart from the obvious health concerns, poor IAQ has a direct impact on employee productivity and decreased retention rates.

According to the World Health Organization (WHO) report, over 10-50% of indoor environments in North America, Europe, Australia and Japan are estimated to be affected by indoor dampness. This figure suggests that mold could be a highly prevalent issue in locations spread across the world.

The question then is how can buildings ensure that the Indoor Air Quality is maintained? The path to a good maintenance program of Indoor Air Quality begins with understanding the standards and the metrics involved so that you can rest assured that the IAQ is maintained at a high quality.

It is important to measure IAQ to determine the extent of the exposure to any pollutants, it will help understand the demographic characteristics affecting the exposure, it will also help evaluate if any remediation and interventions have worked and lastly it is important to the study of IAQ as it related to health outcomes.

The key is to understand the metrics to help you maintain the optimum indoor air quality. Here we take a look at some of these metrics in detail:

1. Ventilation Standards: Commercial buildings have always used building codes that specify the required amount of outdoor air that needs to be let in to help with ventilating the building. With the changes in building construction especially around envelopes, there is an increased need for controlling IAQ.  This is achieved by using mechanical ventilation systems like air ducts, ventilation fans, pollution controlled ventilation systems.  Although there is a lot of variation on acceptable indoor air quality standards in various countries, the most common standard for it is  ASHRAE 62.1.

ASHRAE publishes an elaborate standard for every type of building as well type of room based on various criteria like people outdoor air rate, area outdoor air rate, occupant density etc.

Below is a list of standard for an office building:

2. Perceived Air Quality: (PIAQ) One of the common ways to assess IAQ is to obtain feedback from people since peoples' perceptions of indoor air quality are subjective indicators of the quality of indoor air. They are generally asked to rate air quality on a scale ranging from clearly acceptable to clearly unacceptable. Acceptability of indoor air is affected by various factors including ventilation rate, the emission rates of indoor pollutants, and air temperature and humidity. Perceived air quality is often measured in conjunction with particle matter measurements. This gives a good idea of whether the perception of the occupants is widely deviant from the actual measurement of particulate matter. Ventilation standards generally follow the guideline that indoor air should be perceived as fresh and pleasant by the majority of occupants, so they set a baseline ventilation requirement of around 8 L.s-1 per person to dilute bio-effluent odors to an acceptable level for anyone entering an occupied room from relatively clean air. They then attempt to account for other contaminants by increasing the baseline rate to around 10 L.s-1 per person, although the increase is not based on specific contaminants.

3. Exposure Limits: Exposure limits are concentrations of contaminants to which people should not be exposed.  Certain exposure limits are enforceable by law and others are mere recommendations.  Some other exposure limits apply to a specific set of people and others are for workers in an occupational setting.  No two organizations proposing the exposure limits are the same. The most commonly used exposure limits in the US are published by US Dept. of Labor, Occupational Safety and Health Administration (OSHA). Below is the example of certain chemicals:

4. Particle measurement (PM): According to US Environmental Protection Agency, Particulate matter also referred to as PM or particle pollution is a complex mix of solid and liquid particles suspended in the air. There is a huge concern by the EPA about the particle matter that measure 10 micrometers in diameter or smaller because these particles are inhalable. Particle Number Concentration (PNC) is a commonly used metric while measuring particle matter. Fine particulate matter (PM2.5) is a general term for all small particles found in air measuring equal to or less than 2.5 μm in aerodynamic diameter. This is one of the standards for measuring IAQ particle matter. PM 10 is another standard where the size of the particle is 10 microns or less. In a case study where buildings were measured for particle matter over a period of 6 months, it showed changes from a value within limits in September, increased in October and then decreased in January. This was caused due to external heating temperature. Below is an example of the value of PM2.5 and PM10 evaluated during this period:

5. VOC concentration: Volatile organic compounds, commonly known as VOC, refers to many types of organic chemicals containing carbon, the only exceptions are carbon dioxide and carbon monoxide. Some of the examples are trichloroethylene, benzene, toluene, methyl ethyl ketone, alcohols, methacrylates, acrolein, polycyclic aromatic hydrocarbons, and pesticides. The sources of these chemicals can vary from paint, cleaning products, glues, photocopiers, insecticides, combustion gases, asphalt, tobacco smoke etc. The amount, or concentration, of VOC present in the indoor air is expressed in a variety of units. Commonly used units are parts per billion (ppb), parts per million (ppm), and micrograms per cubic meter (µg/m3). A microgram is one one-millionth of a gram. If the concentration is 1 ppb (or 1 ppm), for every billion (or million) molecules of air there is one molecule of the VOC. If the concentration is 1 µg/m3, then for every cubic meter volume of air there is 1 microgram of mass (weight) of the VOC.

6. Relative Humidity (RH) : Relative humidity is used to measure the atmospheric humidity levels in indoor areas. The best way to measure relative humidity is to note it in a neutral airflow area in each room. These measurements are also taken within the closest location of building occupants, away from doors and windows. RH samples are taken in as many rooms/areas as possible under normal operating conditions. Sources of airborne water vapor are noted. These sources include unvented restrooms, water penetration into crawlspaces/foundations, unvented kitchens, clogged ducts or pipes, and water heaters.

Conclusion:

In conclusion, it is very crucial for buildings to maintain proper Indoor Air Quality (IAQ). This would benefit not only the tenants and occupants by better health and productivity but also ensure that the buildings are retained for longer periods in the marketplace. The way to ensure that IAQ is maintained is to conduct frequent audits and inspections to ensure that IAQ is in accordance to established standards and metrics. Some of the metrics used to measure IAQ are relative humidity, particulate matter concentration, volatile organic matter concentration, perceived air quality, exposure limits etc. Understanding these metrics will help you obtain to IAQ goals.