The Biden administration on Tuesday proposed a first-ever rule to strictly limit the amount of six toxic PFAS “forever chemicals” in Americans’ drinking water. The rule, if finalized, would represent the most significant step toward improving safety at Americans’ taps in nearly three decades — and would impose hundreds of millions of dollars in new costs on American households.

Here’s POLITICO’S guide to what you need to know about the new proposal:

What are PFAS and why are they in the water?

Per- and polyfluoroalkyl substances, or PFAS, are a class of thousands of manmade chemicals that are prized for their ability to resist liquids. They have been used since the 1950s in household products such as nonstick cookware, stain-resistant carpeting, pizza boxes and camping gear, as well as in industrial processes and military firefighting foam that was sprayed at hundreds of bases around the country.

But the strong chemical bond that makes these substances so useful also makes it extremely difficult to break them down, meaning they persist and accumulate in the environment — and inside people’s bodies.

PFAS have been found in groundwater, rivers, lakes and other drinking water sources around the country. There isn’t currently good nationwide data on their occurrence, but research from the nonprofit Environmental Working Group estimates they are in at least 200 million Americans’ water. EPA this year launched a new round of monitoring for 29 PFAS that should eventually give the agency better data on how widespread and severe the drinking water contamination is.

How low can you go?

EPA has concluded that the two best-studied chemicals in the class, PFOA and PFOS, are carcinogens and the agency is proposing to set the drinking water limit at the lowest level that laboratories can detect.

But that limit — 4 parts per trillion — is still two hundred times higher than EPA has said is a safe level of exposure for PFOS, and a thousand times higher than the agency’s safety threshold for PFOA. The Safe Drinking Water Act requires that EPA consider not just health protection, but also the cost and technical feasibility of a limit when deciding how low to set it.

EPA is doing more than the minimum

When the Biden administration committed to setting a drinking water regulation two years ago, it only formally determined that PFOA and PFOS warranted federal regulation under the Safe Drinking Water Act. But the rule proposed by EPA Tuesday goes further, floating limits for four additional PFAS.

That’s important because EPA estimates there are at least 12,000 chemicals in the class, and the treatment techniques that rid water of PFOA and PFOS aren’t always as effective against the newer chemicals that industry shifted toward after they phased out PFOS and PFOA in 2002 and 2015, respectively.

EPA’s proposed approach still falls short of what environmental and public health advocates have asked for: a single limit governing all PFAS. But it represents a major rejection of industry’s argument that each chemical must be dealt with individually. Chemical experts say that approach leaves federal regulators perpetually playing catch-up, with industry shifting to new variations of a chemical once EPA finally has enough information to regulate their current ones.

“What they’re proposing is really the first step towards dealing with the wider array of PFAS,” said Erik Olson with the Natural Resources Defense Council. “No, it doesn’t deal with the whole problem, but it’s taking a good bite out of it.”

Under the Safe Drinking Water Act, EPA must first make a formal “regulatory determination” finding that a contaminant warrants federal regulation, then the law sets a schedule for proposing and finalizing that regulation.

In its proposal Tuesday, EPA is taking these two steps concurrently — proposing both the determination and the limit at the same time.

The rule targets chemicals in active use

The proposal specifically targets the replacement chemicals that industry switched to after evidence of the health harms of PFOA and PFOS mounted. GenX is the chemical that DuPont and its spinoff company, Chemours, used to replace PFOA in Teflon and other products. PFBS is the chemical that 3M Co. used to replace PFOS.

The other two chemicals have a variety of uses, and they are also formed as other substances break down or degrade. PFHxS is used in many consumer products, including firefighting foam, textiles and metal plating, and PFNA is a cousin of PFOA that was used as a processing aid in making plastics.

An EPA staff expert, who was allowed to speak with POLITICO on the condition of anonymity to discuss the agency’s work, said it selected the four additional PFAS out of the hundreds that are in use because of the availability of certain key information.

Specifically, a handful of state-level drinking water regulations gave the agency data proving that these chemicals occur in drinking water, and EPA and other federal agencies have conducted peer-reviewed scientific research showing their health dangers — which include liver and kidney ailments, reproductive and fetal problems and impacts on the immune system. This information is crucial to being able to justify federal regulation under the law.

EPA believes the danger is more than the sum of its parts

EPA is proposing to regulate the four new PFAS using an approach that accounts for the fact that they are often combined as part of a mixture. The agency points to science indicating that the chemicals can be more dangerous in combination than they are on their own, and so EPA is proposing a method called a “hazard index.” Drinking water utilities would monitor for all four PFAS and input the results into a calculation that indicates whether the specific levels and combinations of the four PFAS pose a health danger.

Hazard indexes are a tool frequently used in Superfund cleanups, but have never been used in a drinking water regulation before — although EPA floated the possibility to its outside scientific advisers.

Ridding water if PFAS will cleanse it of other contaminants, too

Many of the technologies that water utilities can use to cleanse their water of PFAS would also reduce other contaminants. These co-benefits are part of the agency’s economic justification for the new proposal.

For instance, Granular Activated Carbon can rid water of the hormones and pharmaceuticals that have emerged as major concerns in drinking water supplies in recent years. It can also remove the organic matter that’s common in river and lake water, but that forms carcinogenic byproducts when mixed with the chlorine that drinking water plants use to treat that water.

Reverse osmosis and other membrane technologies — another option for water plants — can rid water of virtually all contaminants.

The rule “could have amazing effects on the drinking water quality,” Betsy Southerland, a former top scientist in EPA’s water office, told POLITICO last year.

Taxpayers will be on the hook — one way or another

EPA estimates that the regulation will cost $772 million annually to implement, including monitoring source water for the chemicals, installing new treatment technologies at water plants whose water is contaminated, and operating those treatment systems on an ongoing basis. Groups representing water system managers are already suggesting this may be a low-ball estimate and that the actual cost to implement the rule could be far higher. All of those costs would, at least initially, be paid for by customers.

The Biden administration is working on a regulation that could force polluters to foot the bill for cleanups by designating PFOA and PFOS, and potentially other PFAS, as hazardous under the Superfund law. It would also offer water utilities a major legal tool for recouping costs from the entities that sent the pollutants into their water in the first place.

But, taxpayers would still end up on the hook for much of the cost of the cleanups, since the Defense Department is responsible for a large share of the contamination. The military used PFAS-laden firefighting foam for decades at an estimated 700 sites across the country, contributing greatly to groundwater contamination.

It’s not done yet

EPA hasn’t regulated a new contaminant in drinking water since Congress updated the Safe Drinking Water Act in 1996 with strict new requirements for justifying regulations. The one attempt EPA has made to do so was for another military-linked contaminant, perchlorate. The agency issued a determination in 2011 that it warranted federal regulation, but a dozen years later the Biden administration is now fighting environmental groups in court to defend the Trump administration’s decision not to regulate it.

Already, the PFAS proposal has faced headwinds: It took five months, and a last-minute pressure campaign from actor Mark Ruffalo and Sen. Tom Carper (D-Del.), who is close to President Joe Biden, to get it through the White House’s interagency review process and out to the public.

The Defense Department, which faces enormous financial liabilities for cleanups and potentially for health claims from servicemembers, their families and nearby communities, has a record of fighting environmental regulations — including those aimed at PFAS.

Meanwhile, the industry group representing chemical companies, the American Chemistry Council, has come out in opposition to the proposal, arguing that it is based on flawed science.

But, the proposal is getting initial support from at least one key Republican — Sen. Shelley Moore Capito of West Virginia, who has pressured both Democratic and Republican administrations to act on the chemicals.

“After years of urging three consecutive administrations of different parties to do so, I’m pleased a safe drinking water standard has finally been issued for PFOA and PFOS,” she said in a statement. Notably, though, she did not comment on the proposal’s limits for the chemicals that industry is actively manufacturing and marketing.

Even if the administration does finalize the rule, the 1996 law’s requirements could give water utilities, chemical manufacturers or others who might oppose the rule an opening to challenge it in court. The Biden administration’s use of the hazard index calculation — which has not been used in past drinking water regulations and is new to many state regulators and water utilities — could also present legal vulnerabilities.

But the pollution tap is still on

The drinking water limit would require water plants to cleanse water before it flows to taps — but pollution is still flowing into the rivers, lakes and groundwater that they pull that water from.

To stanch that pollution, environmental and public health advocates want EPA to issue regulations that would limit PFAS pollution coming out of industrial smokestacks and discharge pipes. Ultimately, advocates want EPA to ban all non-essential uses of the chemicals.