This story originally appeared on Mother Jones and is part of the Climate Desk collaboration.

Engineer Jim Southerland was hired by the US Environmental Protection Agency in 1971 to join the nascent war on air pollution. He came to relish the task, investigating orange clouds from an ammunition plant in Tennessee and taking air samples from strip mines in Wyoming. Among his proudest accomplishments: helping the agency develop a set of numbers called emission factors—values that enable regulators to estimate atmospheric discharges from power plants, oil refineries, chemical plants and other industrial operations.

By the time Southerland left the EPA in 1996, he was “frustrated and ticked off,” he says, because the numbers he had helped develop were being misused. The original aim had been to paint a broad-brush picture of pollution. Instead, the numbers—meant to represent average emissions from industrial activities—were incorporated into permits stipulating how much pollution individual facilities could release. This happened despite EPA warnings that about half of these sites would discharge more than the models predicted. “These factors were not intended for permits,” says Southerland, now retired and living in Cary, North Carolina.

The number of emission factors used by the EPA since Southerland’s time has proliferated and stands at 22,693. The agency itself admits most are unreliable: It rates about 62 percent as “below average” or “poor.” Nearly 22 percent aren’t rated at all. About 17 percent earned grades of “average” or better, and only one in six has ever been updated. There is a slew of common problems, such as poor accounting for emissions from aging equipment.

“This is what tells you what’s being put in the air and what you’re breathing. You don’t want those numbers to be wrong.”

The upshot: in some cases, major polluters are using flawed numbers to calculate emissions of substances such as benzene, a carcinogen, and methane, a powerful greenhouse gas. Regulators at times are flying blind. The factors color everything we know about air quality and many of the decisions the EPA and state environmental agencies make, from risk assessment to rulemaking.

In an email, an EPA spokeswoman told the Center for Public Integrity that the agency has been working on the problem for a decade. “EPA believes it is important to develop emissions factors that are of high quality and reliable,” she wrote.

Some experts, however, say the agency hasn’t done enough. The unreliability of the numbers has been flagged over a period of decades by the EPA’s own internal watchdog and other government auditors. “This is what tells you what’s being put in the air and what you’re breathing,” says Eric Schaeffer, former head of civil enforcement at the EPA and now executive director of the Environmental Integrity Project, an advocacy group. “You don’t want those numbers to be wrong.”

Accuracy questions

Emission factors are based on company and EPA measurements as well as external studies. They are plugged into equations to estimate total emissions from industrial activities, such as the burning of coal in boilers.

As early as the 1950s, regulators in places like Los Angeles were using emission factors to try to pinpoint the origins of dangerous smog episodes. The numbers allowed them to avoid “time-consuming, expensive testing programs and extensive surveys of individual sources,” according to a 1960 paper by the Los Angeles County Air Pollution Control District.

In 1965, the US Public Health Service—which regulated air pollution at the time—released its first comprehensive list of factors, a document the agency would label “AP-42” in a 1968 update. The EPA, created two years later, kept revising the estimates as they became more widely used in emission inventories depicting pollution levels and sources around the country

The EPA knew early on there were problems with the numbers. In 1989, for example, the Office of Technology Assessment—a now-defunct, nonpartisan science adviser to Congress—reported many US metropolitan areas had not met their goals for controlling smog-forming ozone in part because of inaccurate emission inventories. In 1990 amendments to the Clean Air Act, Congress gave the agency six months to make sure all emissions contributing to ozone formation were assigned up-to-date, accurate factors, and directed the EPA to review the numbers every three years thereafter.