Amid increased scrutiny on a Board of Health proposal to ban so-called “nitrogen loading credits” in sensitive areas, both the Select Board and Planning Board have called for a joint meeting with the Board of Health to address potential impacts on affordable and attainable housing.

Nitrogen loading credits allow homeowners to add additional bedrooms to their properties if they switch to an innovative/alternative (IA) septic system, which removes nitrogen from waste more effectively than conventional systems. Nitrogen in drinking water is linked to health problems, including cancer, as well as environmental harms. The Board of Health is considering eliminating the credits in the wellhead protection district and Hummock Pond watershed, so that homeowners cannot add bedrooms and potentially increase nitrogen pollution.

The new rules would also require all homeowners to switch to an IA system, without receiving any credits, if their existing system fails, if any new construction or structural alterations are proposed, or if the property is transferred from one owner to another.

“If a district is worth protecting at all, it’s worth really protecting. It shouldn’t be kind of protected,” Board of Health chair Ann Smith said.

But nitrogen loading credits are a key way that homeowners increase the availability of housing on-island, and are also critical to offset the high cost of IA systems. Eliminating them in certain areas could negatively impact efforts to increase affordable and attainable housing.

“These added septic regulations would have significant monetary impact on owners that could be prohibitive for the creation of more year-round, or attainable, or even affordable units,” planning director Leslie Snell said.

This has sparked serious concern from both the Select Board and Planning Board.

“We’ve come so far with septic, and now we’re talking like it’s 30 years ago,” Planning Board vice chair Nat Lowell said. “We can’t just come up with something brand new that’s a burden on people when we’re trying to get year-round housing and ways to keep people here that want to live here.”

Multiple Planning Board members criticized the proposed ban.

“I support the idea of the nitrogen [loading credit] bans,” Planning Board member Hillary Hedsges Rayport said. “But I also support and would promote the idea of having a system of waivers for the Affordable Housing Trust’s initiatives.”

The Select Board endorsed a Housing Department recommendation to allow hardship waivers and financial assistance for year-round residents and variances for nitrogen loading credits in exchange for a year-round deed restriction in the wellhead protection district.

This recommendation is supported by the Nantucket Land and Water Council, a local nonprofit focused on protecting Nantucket’s land and water resources.

The Housing Department first made a version of its recommendations last week, but it seemed at the time that they intended the recommendations to apply to the Hummock Pond Watershed as well as the wellhead protection district, which would have put them in conflict with the Nantucket Land and Water Council. The Housing Department has now clarified its stance.

It’s unclear if the Board of Health will agree to allow nitrogen loading credits in the wellhead protection district. Most members did not clearly signal their position at their meeting Thursday.

“We have one aquifer, and once it’s gone, it’s gone,” Board of Health member Kerry McKenna said. “The housing stuff is big, but if you stop and look at the variance [requests] we keep getting, it’s not for three bedroom or four bedroom, it’s for eight bedroom to twelve bedroom. That’s not solving the housing problem. Those are party houses. So, I want to stay focused on what’s going to maintain the health of the community in the long run, and it’s going to be a series of small regulations.”

However, Smith said that the Board of Health is “quite willing” to consider different wording, which could permit the credits. She also suggested the new wording could be incorporated without a joint meeting, if the Board of Health supports it.

Some areas, including the Madaket and harbor watershed regions, are already subject to bans on loading credits.

The Board of Health is also considering another regulation that could be even more controversial: a ban on synthetic turf. Last month, the School Committee voted 4-1 to replace the current football field with a turf field. Days later, the Board of Health voted to schedule a special meeting to consider a ban.