Stuck at home because of the coronavirus pandemic, people who found themselves sitting on savings from canceled vacations and other cutbacks built pools in record numbers last year to make quarantine more enjoyable. Then a fire at the end of last summer caused a shutdown at the plant that produces most of the country's supply of chlorine tablets. This year, the arrival of Memorial Day and pool season has industry experts warning of a shortage that threatens to disrupt backyard plans from coast to coast. "There is a very, very good possibility that we're going to run out of chlorine tablets," said Rudy Stankowitz, a swimming pool consultant and educator with more than 30 years of experience in the industry. "People should start looking into alternative methods of sanitizing swimming pools." The shortage, which Mr. Stankowitz described somewhat hyperbolically as a "poolocalypse," will affect residential pools more than public ones, which typically use different forms of chlorine. There are significant public health implications. Chlorine tablets are used to sanitize pools and to keep them free of algae and bacteria. Inadequately sanitized water can become turbid, making it a safety risk and a leading cause of drownings, Mr. Stankowitz said. "If somebody falls in, you might not see them, or they can get disoriented and not figure out how to get out," he said. "There's even been cases where pools were green enough that they looked like a lawn." The chlorine shortage has caught some new pool owners like Stephanie Winslow off guard. Ms. Winslow was accustomed to spending hot days with her daughter and friends at a local public pool. When it closed because of the pandemic, she bought an aboveground pool for the cattle farm where she lives with her family in Philippi, W.Va. "We get a lot of use out of it," Ms. Winslow, an online college financial aid specialist, said. "We pretty much use it every day." This spring, Ms. Winslow bought a month's supply of chlorine tablets at Walmart for $26. After hearing about the shortage, she returned to buy enough to last through the summer, she said, but there were none available at Walmart or Ace Hardware. On Amazon, a bucket of tablets — the same size that had cost her $26 a week earlier — was listed for nearly $170.