More than 500 old power plants and 17,000 other aging factories and refineries will be allowed to make significant equipment upgrades without having to install costly pollution control devices, according to a ruling issued this week by EPA.

EPA's new rule permits plant operators to make upgrades worth 20 percent of the facility's value without adding new controls, such as scrubbers in their stacks.

The Clean Air Act requires new plants and utilities to install the best available pollution control technology, but plants and refineries built before 1970 are exempt from the requirement unless they make extensive and costly improvements that boost productivity and pollution.

Now older plants can replace equipment such as a turbine or boiler without installing pollution controls, provided the cost of the upgrade does not exceed 20 percent of the replacement value of the entire facility. The plant must still comply with overall pollution permit limits and other state and federal programs for pollutants.

EPA's decision was hailed as a sensible, cost-effective decision by industry groups and another environment retreat by the Bush administration, according to green groups.