Source control is the EPA's first and most effective strategy for improving indoor air quality. The agency frames IAQ around three approaches — source control, improved ventilation, and air cleaning — and is explicit that eliminating or reducing a pollutant at its source is usually the most effective, and the most cost-efficient, because ventilating contamination away raises energy bills while leaving the source in place.
In practice, source control means dealing with the cause rather than masking it: venting combustion appliances and range hoods to the outside, fixing the moisture that feeds mold, sealing or enclosing materials that off-gas, storing solvents and paints away from living space, and choosing low-VOC products. None of it is glamorous, and none of it is something a company can sell you in a box — which is exactly why it gets skipped in favor of gadgets.
For a homeowner, source control is the test of whether you're getting honest advice. A contractor who asks where the problem is coming from — a back-drafting water heater, a damp crawl space, a new-construction off-gassing phase — and addresses that first is following the EPA's own hierarchy. One who jumps straight to an ionizer or a UV lamp without ever finding the source has the order backwards. Filtration and ventilation are the next layers, not the first.