The incident occurred at around 7 am at a household scrap collection business in Me Son Village, Thuong Phuc Commune, according to local authorities.
A local official said a valve on a gas cylinder was accidentally broken during scrap collection, causing a suspected toxic gas leak that affected the business owner and nearby residents.
All 40 people were taken to Thuong Tin General Hospital for treatment. Two patients showing signs of serious illness were transferred to Bach Mai Hospital for further monitoring and treatment.
Authorities have temporarily cordoned off several nearby homes while health officials assess air quality and determine when residents can safely return.
Local officials have also visited the hospital to check on the patients.
The incident occurred in a temporary structure used by a local scrap collection business. Authorities are continuing to investigate the exact cause of the leak.