Fears of a chemical incident were sparked this morning after vomiting residents were rushed to hospital and a South London street was put on lockdown. A total of 24 people were evacuated from their homes in Alwold Crescent, Lee, after neighbours reported the smell of an unknown substance. Seven people were treated at the scene for nausea and vomiting - while two children were taken into hospital for further treatment as a precautionary measure. Scotland Yard said the incident was not believed to be crime-related. Witnesses said emergency services appeared to be investigating the sewage system. Taxi driver Ian Crouch, 39, told The Sun Online: "I could see the emergency services outside my house. "When I spoke to a policeman he said there were people complaining of irritation, who had been throwing up all morning. "They've been rushed into hospital." The 39-year-old added: "He said they were investigating whether it is terror-related, but told us not panic. "It sounds like we might get evacuated from our homes. "They think it's coming from the sewage system." Wayne Higgins, 50, said that he and his neighbours had been feeling unwell for the last few days. "The fire brigade have been here checking if there are chemicals or toxins. For the moment we don't know when we will be allowed back in - not until they find out the cause of the problem. "We just assumed everyone had a cold or something was going round." Kevin Dobbins, 31, who was also asked to leave his home, said: "I haven't smelled anything or felt the effects myself. I know there were neighbours who were experiencing sickness. "They did pull us all together and had a meeting with us saying you will have to leave the area and it could be throughout the night and in to tomorrow. "They are still uncertain. They have done checks and they've ruled out a few different things." The cause of the incident remains unclear. London Fire Brigade (LFB) said the smell of the chemicals had swept the area but found no raised levels of chemicals or toxins. A spokeswoman for LFB said: "There was a smell of an unknown chemical at an address in the area and we are investigating that. "Some people have been removed as a precaution and others have been taken to hospital - also as a precaution." Emergency crews were called to the scene at 10am this morning amid fears locals may have suffered an allergic reaction. Residents at one end of the crescent were asked to leave their homes at around 11.30am and a cordon was erected. Five fire engines, paramedics and police are all on hand to deal with the incident. LFB has now handed the investigation over to the Gas Authority. A spokeswoman for Southern Gas Network (SGN) said: "Our safety checks have just been completed and there is no gas leak in relation to the properties which were evacuated earlier by the fire brigade. "The investigation into the cause is ongoing."