A major incident was in place on Wednesday after authorities said a spill of sodium cyanide into a West Midlands canal posed a serious health risk to the public. Members of the public are being advised to avoid a 12-mile stretch of canals and towpaths centred in Walsall after the toxic spillage, understood to have happened on Monday. The risk is to people and pets who have direct contact with the canal water. According to the UK Health Security Agency, sodium cyanide can cause headache, nausea, dizziness, nervousness, confusion, changes in heart rate, drowsiness, fitting, vomiting, low blood pressure and loss of consciousness. Sodium cyanide consists of “white crystal-like solids with a faint almond odour”. It is, the agency said, “used in industry for metal cleaning, plating and extraction and photography”. Walsall council said it was notified of a spillage in the canal at Pleck, which stretches for more than a mile from Spinks Bridge to Birchills Street Bridge, on 12 August. A major incident was declared the following day. On Wednesday, dozens of dead fish could be seen floating in the canal after the cyanide spillage. Garry Perry, the leader of Walsall council, said the priority was the safety of local people. “I share their concerns and hope to see this incident resolved as soon as possible. We are working closely with our partners to manage this situation, which has been declared a major incident. For your own safety please avoid this area of the canal and its towpaths,” he said. Perry told the BBC that it was “a very live and ongoing investigation”. He added: “Anything like this is serious – particularly where the potential risk to health and safety of animals and humans is always going to cause alarm.” Walsall council said the source of the spillage had been identified and stopped and an investigation was under way into its cause. One expert said the spillage was “a concern” and it could take days to remedy. Jonathan Paul, senior lecturer in geosciences at Royal Holloway, University of London, said: “Uncontrolled spills into waterways present a major ecosystem hazard. It’s also a risk to the public water supply if the canal water meets leaky sanitation lines. Then the greatest risk for people would be encountering the toxic canal water, for instance via touch.” He said a high concentration of sodium cyanide would result in “classic poisoning symptoms” of nausea, weakness, aches, and potentially a loss of consciousness. “But that is entirely dependent on how much is ingested and the concentration of the chemical.” The council posted a map showing the canals the public should stay away from. It included waterways from Walsall lock flight to lock flights at Rushall, Ryders Green and Perry Barr. Anyone exposed to the canal water in the area who is feeling unwell has been advised to seek health advice through calling 111 or, in an emergency, 999. The council said anyone who had caught fish from the canal should not eat them. Drinking water is not affected. The spillage is being treated as a major incident, involving police, fire, ambulance, councils, the Environment Agency, the UK Health Security Agency (UKHSA), Severn Trent Water and the Canal and Rivers Trust. A sampling and testing programme has been established by the Environment Agency with the aim of reducing the affected area as soon as possible, the authorities said.