Police were called at 18:17hrs on Thursday, 2 November to Walpole Road, Walthamstow E17 after a man was sprayed with a noxious substance.
Officers attended along with the London Ambulance Service and the London Fire Brigade.
The victim, believed to be in his 30s, was treated at the scene and then taken to an east London hospital for treatment. His condition is described as life-threatening at this stage.
It was reported that there were two suspects on a moped, and this was an attempted robbery.
No arrests; enquiries continue.
If you have any information about this incident, call Waltham Forest Police on 101 or Crimestoppers on 0800 555 111.