Pollen, dust, fungi, industrial pollutants, and general vehicle exhaust are common outdoor sources of indoor air pollution. Other sources include exhaust from vehicles on nearby roads or in parking lots or garages, loading docks, odors from dumpsters, unsanitary debris near outdoor air intakes, and cigarette smoke from office workers now required in most cities to take their cigarette breaks outside of building entrances. Another source is exhaust from the building itself or from neighboring buildings that is re-entrained, or drawn back into the building.
However, the most significant newly "discovered" out-of-building source in office buildings, and indeed all buildings, is soil gas vapor. Such vapors rise through groundwater into subsurface soil, and can be compressed just below building foundations to generate relatively high levels of interior pollutants. Contaminants include those from leakage of underground storage tanks, contaminants from previous uses of the site (e.g., landfills or historic industrial uses) whose past clean-ups were insufficient; pesticides used on the grounds outside the building, and occasionally from radon. The very recent and vigorous regulation of soil gas vapor by the state in New York, which includes new tools for calculation of indoor contaminant levels of potentially health-threatening magnitude based on subsurface levels, is discussed below. Federal regulation of soil gas vapor is at the draft stage as of this writing, but is expected within the next year.