The worst school disaster in U.S. history, the New London School fire began with a natural gas leak that spread through the building's crawlspace, which ran the length of the school. A spark ignited the school. The building collapsed and almost 300 were killed. Since the gas was odorless, no one was aware of the leak. As a result, the U.S. and Canada now require that "any combustible gas within a distribution line and transmission line (exceptions noted in the rules) must contain odorant at the level of 20% (1/5) of the lower explosive limit so that a person with a “typical” sense of smell can detect it."
