Two people have been taken to hospital and 33 others treated by paramedics after being taken ill at an industrial estate.
Paramedics and firefighters were alerted to the incident at Newlands Industrial Park in Somersham, Huntingdon, Cambs, at 9.40am today.
The fire service said the alert could be related to crop spraying in a nearby field.
Businesses on the site are being advised to close windows and doors.
The East of England Ambulance Service deployed its hazardous area response team to the site.
Spokesman Gary Sanderson said: "A total of 35 casualties have been assessed by HART and land paramedics at the scene, two of which, both men have been taken to Hinchingbrooke Hospital for further assessments.
"Throughout the incident, the three emergency services worked very closely together to treat the casualties and ascertain the cause of their sudden illnesses this morning."