A system for determining indoor air contaminant levels independent of outdoor contaminant levels, comprising: an air contaminant monitoring system comprising: a first air contaminant sensor for collecting air contaminant levels from at least one partially enclosed area that is served by at least one return air handling unit that mixes at least a portion of building air returned to the air handler air into supply air; a second air contaminant sensor for collecting air contaminant levels from the supply air outlet of said return air handling unit; and at least one processor configured to: determine a return air contaminant fraction from the level of return air inlet contaminants of said return air handling unit that are present in said return air handling unit supply air outlet and the level of the air contaminants in the return air inlet; and process at least one indoor air contaminant parameter from: a magnitude of the difference between the air contaminant levels of said partially enclosed area and said supply air outlet of said return air handling unit feeding said partially enclosed area; and said return air handling unit return air contaminant fraction. an air contaminant monitoring system comprising: a first air contaminant sensor for collecting air contaminant levels from at least one partially enclosed area that is served by at least one return air handling unit that mixes at least a portion of building air returned to the air handler air into supply air; a second air contaminant sensor for collecting air contaminant levels from the supply air outlet of said return air handling unit; and a first air contaminant sensor for collecting air contaminant levels from at least one partially enclosed area that is served by at least one return air handling unit that mixes at least a portion of building air returned to the air handler air into supply air; a second air contaminant sensor for collecting air contaminant levels from the supply air outlet of said return air handling unit; and at least one processor configured to: determine a return air contaminant fraction from the level of return air inlet contaminants of said return air handling unit that are present in said return air handling unit supply air outlet and the level of the air contaminants in the return air inlet; and process at least one indoor air contaminant parameter from: a magnitude of the difference between the air contaminant levels of said partially enclosed area and said supply air outlet of said return air handling unit feeding said partially enclosed area; and said return air handling unit return air contaminant fraction. determine a return air contaminant fraction from the level of return air inlet contaminants of said return air handling unit that are present in said return air handling unit supply air outlet and the level of the air contaminants in the return air inlet; and process at least one indoor air contaminant parameter from: a magnitude of the difference between the air contaminant levels of said partially enclosed area and said supply air outlet of said return air handling unit feeding said partially enclosed area; and said return air handling unit return air contaminant fraction. a magnitude of the difference between the air contaminant levels of said partially enclosed area and said supply air outlet of said return air handling unit feeding said partially enclosed area; and said return air handling unit return air contaminant fraction.