A chemical plant near Sarnia evacuated workers Wednesday afternoon after a hydrogen sulphide leak, a plant worker said. Staff at the Nova Chemicals site in Corunna, south of Sarnia, were told to evacuate at about 4:45 p.m., the plant source, who asked not to be identified for fear of reprisal, told the Star and Global News. An alert issued by the nearby First Nations community of Aamjiwnaang stated the chemical spilled was hydrogen sulphide. It's not clear when the leak began. The same source said Nova staff were told Wednesday morning that a leak "happened last (Tuesday) night and (was) still not contained." The company, which employs about 500 people at the plant, called an all-clear just after 6 p.m. A spokesperson for the Ontario Ministry of the Environment, which must legally be told about any chemical spills, didn't say if Nova notified the government of the incident but said it's now aware of an "alarm" at the plant. "The ministry has reached out to Nova Chemicals," Environment spokesperson Gary Wheeler said via email. "Nova will provide information to us as soon as possible if there are any environmental issues that require ministry involvement. Ministry staff will respond as soon as possible, if necessary." Nova didn't respond to questions about when the leak began, if the leak affected the surrounding community, whether it has done air monitoring, or what results that air monitoring may have shown. Last month, a joint investigation by the Star, Global News, National Observer, the Michener Awards Foundation and journalism schools at Ryerson and Concordia universities revealed a troubling pattern of secrecy and potentially-toxic leaks in the Sarnia area. There are 57 polluters within 25 kilometres of the city registered with the Canadian and U.S. governments. The investigation also raised questions about whether companies and the provincial government are properly warning residents of Sarnia and Aamjiwnaang when potentially toxic substances are leaked. Hydrogen sulphide, also known as sour gas or H2S, has a rotten-egg odour. At high enough concentrations, it can paralyze the human sense of smell and cause death. Workers at Nova's plant in Corunna received written instructions to wear half- and full-respirator masks in certain areas when they arrived Wednesday morning, the source said. Nova didn't answer questions about how high H2S levels typically reach before staff are asked to wear protection. "These communications are shared twice a shift with the Corunna Site workforce (by email, safety meetings, etc.) to ensure they are aware of the changing personal protective equipment requirements in isolated areas during the shutdown and restart of facilities," said Nova spokesperson Meaghan Kreeft via email. Aamjiwnaang sent out a public alert about the leak 5 p.m. The City of Sarnia didn't send notice of the incident to its residents.