More than 470 people are suing BP for $1 billion, saying they became sick due to a 15-day gas leak at its refinery in Texas City. The Galveston County Daily News reports that BP says mercaptan leaked from a storage tank in November 2011. Mercaptan is a pungent, harmless gas often added to natural gas so that the odorless, flammable substance can be more easily detected. Attorney Anthony Buzbee says the leak also contained sulfur dioxide, a toxic gas. BP has declined to comment. The refinery has since been sold to Marathon Petroleum. In 2005, 15 people were killed in an explosion at the same refinery. More than 50,000 people have sued BP saying they were sickened by another leak at the refinery in 2010.