The U.S. Environmental Protection Agency last month proposed the first federal limits on PFAS — manmade “forever chemicals” linked to cancer, organ damage and other health issues — in the nation’s drinking water.   The proposed regulation, which is not final, would require water systems to reduce levels of six of the most studied types of PFAS to the lowest levels that can be reliably measured with testing. There are about 12,000 kinds of PFAS (per- and polyfluoroalkyl substances) chemicals, which in consumer products are probably best known for their use in nonstick cookware, Gore-Tex outerwear and stain-resistant carpets and rugs. The two most-studied PFAS — PFOA and PFOS — are no longer allowed in products in the United States. But because the chemicals are resistant to breaking down, they linger in the environment.