Emergency services were called just before 5pm on Monday (September 8) and the check-in area was cleared as a precaution.
Police said specialist officers carried out a full search but “no trace of any adverse substance was found”.
The London Ambulance Service said 21 patients were treated at the scene.
One was taken to hospital, while the others were discharged.
The Metropolitan Police confirmed none of the injuries were life-threatening or life-changing.
The London Fire Brigade sent crews from Feltham, Heathrow, Wembley and surrounding stations. At its peak the incident saw officers, firefighters and paramedics working together at the scene.
By 8.27pm, Heathrow confirmed on X that Terminal 4 was “safe to reopen” and it was working to ensure flights departed as planned.
Earlier, passengers had been advised not to travel to Terminal 4 and trains were unable to stop there. Footage shared on social media showed travellers waiting outside, some wrapped in foil blankets.
Transport Secretary Heidi Alexander said she was monitoring the situation.
The cause of the incident remains under investigation.
