WASHINGTON (AP) — The impact on poor communities of cutting Chesapeake Bay pollution through credit trading is being questioned in a report released Wednesday by a nonprofit policy analysis group.

The Center for Progressive Reform said that the report finds that even if trading cuts overall pollution, it might still have a negative impact on low-income and minority communities.

Pollution trading allows some polluters to buy credits for cuts made by others. Maryland, Virginia, Pennsylvania and West Virginia already have state programs, but a bay-wide program does not exist.

Supporters argue a trading program could encourage additional cuts by some polluters who could then sell credits. Farmers, for example, may be able to cut pollution more easily than a sewage treatment plant. However, the authors said if the program is not properly monitored, buyers of credits could create pollution hot spots by not reducing pollution in their area.

"We should judge a trading regime by whether it will actually contribute to cleaning the bay, and whether it will be fair to all," said co-author and CPR President Rena Steinzor.

Credit-generating activities can also include projects that benefit the bay and the community, the report said.

Municipalities can generate credits through expanding green spaces that will reduce runoff of polluted stormwater while also providing space for exercise and increasing property values.

Another report released in January by the Senior Scientists & Policymakers for the Bay said the concept has promise, but must be vigorously monitored to guard against fraud and to ensure large traders don't have an advantage over smaller groups.

A study that was presented in May to the Chesapeake Bay Commission, which advises state legislators in Maryland, Virginia and Pennsylvania, said trading could cut the costs of restoring the Chesapeake Bay as much as 80 percent. However, the commission said the study also highlighted the need to ensure trading actually delivers cuts.