A deadly form of soot pollution from U.S. refineries has gone unregulated for decades because of a dispute between the U.S. oil industry and federal environmental officials over how to measure it, according to documents from the Environmental Protection Agency (EPA).

The delay in addressing so-called condensable fine particulate matter emissions means this pollutant is being released by scores of facilities across the country unchecked, adding to a slew of other contaminants from oil refineries that researchers say take a disproportionately large toll on the health of poor and minority communities living nearby.

The absence of a federal standard has led at least one regional air quality regulator in California to attempt a crackdown on these emissions, an effort that has sparked litigation from oil refiners located there.