• Indoor Generated Pollutants

For pollutants released indoors, increasing the ventilation rate will generally reduce the indoor concentration. However, the air pollutant concentrations in a given space depend on several factors other than ventilation air flow rate. Mixing of outside air in the occupied space is one such factor. The mixing of outside air is not necessarily complete, and the concentration of pollutants at the breathing zone may vary significantly depending on the air distribution pattern and the locations of pollutant sources. The indoor pollutant generation rate (source strength) is usually not constant. Another factor is pollutant adsorption by room surfaces during high concentration periods and desorption again into the air during low concentration periods. Also, indoor pollutant source strengths are highly variable among buildings, and considered the biggest cause of the variation in pollutant concentrations among buildings (Turk et al. 1987).

In many buildings, ventilation rates are not constant. For example, ventilation systems may not operate at night, and rates of ventilation during operation may change with internal heat loads or with outdoor air temperature. Pollutant concentrations may not reach equilibrium until several hours (if ever) after ventilation rates stabilize. Thus, the indoor air pollutant concentrations are also dependent on the operating schedule of the ventilation system. For information on sources in air handling systems, see (Björkroth et al. 1998, Seppänen 1999).

• Pollutants only from outdoor air

If a pollutant is not released indoors but is present in the outdoor air that enters the building, increasing ventilation rates may increase the indoor concentration. This occurs when the pollutant is removed from indoor air by reaction with surfaces (e.g. ozone) or by air cleaning (e.g. outdoor air particles). If there is no indoor pollutant removal, the indoor and outdoor concentration will be equal, regardless of the ventilation rate (e.g. carbon monoxide when there are no indoor sources). Other important factors are the location so fhte outdoor air intakes relative to outdoor pollutant sources including exhaust air outlets and pollutant sources in the air handling system.

• Pollutants released indoors and also present in outdoor air

For pollutants released indoor and also present in the outdoor air, increasing the ventilation rate can decrease or increase the indoor concentration. What actually happens will depend on the indoor release rate, the outdoor concentration, and the extent of indoor pollutant removal by indoor deposition, chemical reactions or by air cleaning.

• How does ventilation rate affect indoor VOC concentrations

Volatile Organic Compounds (VOC) - Direct relationships between VOC concentrations and ventilation may be obscured by a number of factors in addition to chemical reactions in the indoor air. There may be overriding temporal and spatial variations in source strengths. For example, concentrations of compounds that are solvent constituents of products used intermittently or sporadically and occasionally in large quantity would not be expected to exhibit strong inverse relationships with ventilation. In a recent study (LBNL-49535) this was apparently the case for acetone, isopropanol and 2-butoxyethanol. Ventilation and local airflow rates may influence VOC emissions from wet products applied to surfaces and from building materials, furnishings and other solid sources. The sorption of VOCs onto surfaces in the building and their later release when bulk air concentrations decline also directly links VOC emissions with ventilation. For some common VOC/material combinations, this effect is predicted to be relatively large (Zhao et al., In press). These processes, plus imperfect air mixing and air chemistry, reduce the effectiveness of ventilation for controlling VOC sources.

Temporal and spatial variations in cleaning products likely obscured the relationship between concentration and ventilation for solvents. For other compounds, it is likely that effective emission rates increased with ventilation due to their re-emission from sinks. Homogeneous chemistry also may have altered the relationship between ventilation and concentrations of some reactive and product compounds. These results emphasize the importance of source control for limiting the concentrations of VOCs in buildings. Control procedures include use of low emitting materials to finish and furnish interiors, use of low-emitting cleaning products, and avoidance of products containing highly reactive chemicals.

Figure 3. Indoor minus outdoor VOC concentrations in four AHUs on 7 sampling days versus normalized AHU airflow rates. (LBNL-49535)