Lead in drinking water usually comes from plumbing — lead service lines, old solder, and some fixtures — not from the water source. It’s especially harmful to children, and the only way to know if it’s at your tap is to test. Here’s how it gets into water and what reduces exposure.

Acid rain is precipitation made more acidic by air pollution, mainly sulfur dioxide and nitrogen oxides from burning fossil fuels. It has damaged lakes, forests, and buildings, but emissions cuts have greatly reduced the problem in many regions. Here’s how it forms and what’s changed.