We commonly use the term "humidity," but the proper term is relative humidity (rh). Relative humidity is the percent of moisture in the air relative to the moisture the air can hold at a specific temperature. When you see 50 percent rh it means that the air holds 50 percent of the moisture that it is possible to hold at that temperature. The higher the air temperature, the more moisture it can hold. This means that air at a temperature of 80 degrees F and 50 percent rh holds more grains of moisture than the same amount of air at a temperature of 60 degrees and 50 percent rh.

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Be sure to provide the amount of ventilation air set by codes. Insufficient ventilation can create health problems.

  • Carbon monoxide (CO) is a deadly gas that is a result of incomplete combustion. It is produced by propane, natural gas, or kerosene heaters that are faulty or unvented. It is produced by cars, trucks, and buses, and can be pulled in from the street if the outside air intakes are not properly located.

  • Carbon dioxide (CO2) is exhaled as we breathe. It can build up in crowded areas that are not properly ventilated. CO2 itself is not hazardous, but as it increases, the oxygen level decreases. Low oxygen levels are dangerous.

  • Many cleaning solvents, paints, colognes, adhesives, and other products give off hazardous or unpleasant fumes.

  • Copy machines produce ozone, which can be hazardous.

    Adapted from Indoor Air Quality from the Indoor Environment Technician's Library series,

    Turning Vanes

    Square throat elbows must have turning vanes to reduce dynamic loss.

    Double-wall turning vanes were commonly used in the past. However, research has shown that these are less effective than single-wall vanes.

    Single-wall turning vanes produce the smallest amount of dynamic loss. A single-wall turning vane should have a trailing edge. This will further reduce dynamic losses.

    Adapted from Airflow in Ducts by Leo Meyer, from the Indoor Environment Technician's Library series,

    Publication date: 09/26/2005