WASHINGTON (Reuters) - Air quality in the Arkansas neighborhood where an Exxon Mobil Corp pipeline leaked thousands of barrels of Canadian crude has improved but was still unhealthy where workers were cleaning it up, the company and the U.S. environment regulator said on Thursday. Readings of air quality "are below levels likely to cause health effects for the general population with the exception of the cleanup areas where the emergency responders are directly working," the Environmental Protection Agency said. "As cleanup continues, contaminant levels continue to decrease," it said. ...