INDOOR VERSUS OUTDOOR AIR

This narrative will discuss indoor air quality (IAQ) inside occupied structures, sources and behavior of contaminants, and methods to identify the bad air affecting building occupants. While the focus is on indoor, not outdoor air quality, it should be understood that outdoor air can affect indoor air quality (IAQ), especially when there is something bad in it. Moreover, the lack of outdoor air, or fresh air, can also affect IAQ. Another source of "bad air" that gets indoors are soil-gas vapors - volatile chemicals from the soil migrating upward, penetrating through building foundations thereby affecting IAQ. The vapors in soil-gas may be from a man-made source (e.g., leaks from underground gasoline (BTEX) tanks) or they may be naturally occurring, (e.g., methane, hydrogen sulfide and radon). Therefore, contaminants in indoor air can originate from outdoors, below surface, and of course from indoor sources. In summary,

THINGS IN THE AIR

The air as we know it, is a mixture of chemicals. It is mostly oxygen and nitrogen gas, approximately 20% and 78% by volume respectively, which makes up roughly 98% of the volume of air we breath. The oxygen (O2) and nitrogen (N2) gas mixture has an atomic weight of approximately 60 grams per mole. (Please note - the numbers mentioned herein are not exact for any situation, but are relative for purposes of this discussion and to provide perspective. Exact volumes, flow rates, and mass of constituents in air can vary significantly depending upon internal conditions and external contributing factors at the time and place where they are measured.) While air is approximately 98% oxygen and nitrogen, the last 2% can be significant because it represents 20,000 parts per million (ppm) by volume of other stuff. Quite a bit of this 20,000 ppm is made up of common gases (e.g., carbon dioxide), some is exotic gases (e.g., argon) and the rest is man-made volatile chemicals (e.g., chlorofluorocarbons). 50-years ago, readily available scientific instruments could measure contaminants in air down to approximately 0.5% by volume. 20-years ago we could measure contaminants down to less than 1-ppm (i.e., 0.0001% by volume). Current technology now allows us to measure chemicals in air to well below 1-part per billion (ppb). Based on our current knowledge, there are a lot of chemicals in the air besides oxygen and nitrogen.

In addition to gases and volatile chemicals, the air is also a medium for aerosols, both liquid and solid particulate aerosols. These particulates are dispersed in the air and their dispersal is neither constant nor uniform. The most common aerosol dispersed in air is water droplets - we see them as clouds.

The air also has physical parameters that may be relevant when evaluating IAQ, e.g., temperature, humidity, velocity, pressure and flow patterns, and each one of these physical parameters is relative to each other. For example: warm air rises, which may create a below-ambient pressure on the ground floor of a tall building, which enhances soil-gas vapor intrusion through the foundation and into the first floor's occupied space, negatively affecting IAQ.

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