An explosion and fire last night around 6 pm at the MarkWest Energy natural gas processing plant in Chartiers (Washington County), PA sent four people to the hospital–carried there by helicopter. All of them remain hospitalized, and one of them is, sadly, in critical condition. The explosion happened near “two temporary tanks that were onsite for routine maintenance,” according to a MarkWest statement. The tanks hold, “liquid ethylene glycol plus hydrocarbons”–used to clean incoming raw natural gas. The PA Dept. of Environmental Protection is on location today to determine what happened and why–and to ensure there have been no negative impacts to the environment.