Most days, when she leaves her house in Fort Bend County, south of Houston, she does so wearing a mask. Donna Thomas, a 63-year-old environmentalist, doesn’t take the precautionary measure because of COVID-19 concerns. Instead, she aims to protect herself from the gray plume of smoke—a “menacing storm cloud,” as she puts it—that sometimes wafts from a coal-fired power plant about fifteen miles southwest of the neighborhood where she’s lived since elementary school. Thomas, who has asthma, says she feels an intense weight on her chest when she spends too much time outside, and she sometimes has to retreat indoors because the air reeks of “trash and sewage.” As president of the small air-monitoring neighborhood council Fort Bend County Environmental, Thomas has worked for more than…