The Environmental Protection Agency announced Tuesday it has denied a petition by environmental groups to regulate concentrated animal feeding operations like factories under the Clean Air Act.

EPA Administrator Scott Pruitt, in a letter to petitioners, acknowledged livestock are potential sources of air pollutants. The agency, however, doesn’t have a reliable method for estimating animal emissions. Until it does, new rules could be unjustified and ineffective, according to Pruitt.

“Once the agency has sufficient information on CAFO emissions, it will determine the appropriate regulatory approach to address those emissions,” he stated.

The EPA decision, posted in the Federal Register, answers a petition filed in 2009 by The Humane Society of the United States and other environmental groups.

The groups sought to bring concentrated animal feeding operations under Section 111 of the Clean Air Act. The section requires stationary sources of air pollution to adopt the “best system” for reducing emissions. The groups said farms with a large number of animals harm human health, poison the environment and cause climate change.

The EPA says it can’t judge a farm’s contribution to air pollution based on the number of its animals. Weather, geography, management practices and other factors affect livestock emissions, according to EPA.

Pruitt said the EPA will finish work leftover from the Obama administration and develop a reliable method for estimating farm emissions. The agency then will be able to identify how to control them, he wrote.

“Until the EPA more fully understands the level and sources of emissions of different pollutants, it would be premature for the agency to decide which (Clean Water Act) regulatory tool or tools could be most effectively applied to protect public health and welfare from these emissions,” according to Pruitt.

Other petitioners included the Waterkeeper Alliance, Sierra Club, Environmental Integrity Project, Friends of the Earth and the Association of Irritated Residents. The groups sued the EPA in 2015 and again this year alleging the agency was taking too long to make a decision.

The Obama administration also took the position that it needed a more accurate way to estimate emissions before invoking the Clean Air Act, according to court records.

Some of the same environmental groups successfully sued the EPA in another case related to animal agriculture. The D.C. Circuit Court of Appeals ordered the EPA to require farms to register with the National Response Center if their animals give off more than 100 pounds of ammonia or hydrogen sulfide in 24 hours.

The court is expected to finalize the order Jan. 22. EPA has offered worksheets to help farms calculate emissions, but says there is no generally accepted way to make the estimate.