Tucked inside the Trump administration’s plan to ease carbon dioxide limits is a change that could breathe new life into scores of aging coal-fired power plants.

Experts say the shift could also unleash an additional tens of thousands of tons of nitrogen oxide emissions into the air each year.

The proposal would permit upgrades to old power plants without triggering an existing legal requirement to install costly pollution control systems at the same time. The upgrades could extend the lives of those plants by making them more reliable or cheaper to run.

For coal-fired power plant owners, “this proposal seems to want to let them have their cake and eat it too: to improve their efficiency, run more often and not trigger” the requirements, said Andres Restrepo, a staff attorney with the Sierra Club.

More than a third of American coal-fired units lack modern controls to pare smog-forming nitrogen oxide emissions, according to a Bloomberg News review of Environmental Protection Agency data.

Some of those plants went into operation in the 1950s and 1960s, predating advanced catalytic-reduction equipment and regulatory requirements compelling its use.

All told, at least 33 percent of U.S. electricity generated by burning coal last year was not subjected to advanced pollution controls, according to EPA data.

For years, owners of those aging facilities have faced a major challenge: The plants might benefit from equipment replacements and upgrades that would make them more reliable and less costly to run — and better able to compete with natural gas-fired units. Yet any change that would bolster nitrogen oxide emissions by 40 tons a year — perhaps just because the plants would run more as a result of the upgrades — would also trigger requirements to install modern controls to keep those pollutants at bay.

President Donald Trump’s EPA is offering a revision as part of its proposal to replace an Obama administration regulation slashing carbon dioxide emissions across the nation’s electric grid with modest requirements for efficiency upgrades at individual power plants.

At issue is the federal government’s so-called New Source Review program, which allows power plants, refineries, factories and other industrial facilities to keep running with existing pollution-control equipment — even if it has become outdated — so the systems don’t constantly need upgrades.

Under the New Source Review program, requirements for better pollution-control systems are triggered whenever the sites undergo construction, renovations or some operational changes expected to significantly boost annual emissions.

The Trump administration’s proposed changes would make it harder to trigger the requirement by adding an additional test: insisting that emissions also go up on a short-term, hourly basis — in addition to the annual surge.