The site in Freshfields, St Leonards, was evacuated after people fell ill and a "strong smell of ammonia" was reported, at about 08:30 BST.
Emergency services cordoned off the area and urged nearby residents to close their windows and doors.
Eleven members of staff were treated at the scene and two people were taken to hospital.
East Sussex Fire and Rescue Service think the injuries were caused by fumes from an ammonia-based substance.
Fire crews are monitoring gas levels which they said "were reducing".
South East Coast Ambulance Service (Secamb) said a "routine precautionary decontamination" was being carried out now that the incident had been "stood down".
Patients were treated and assessed at the site for sore eyes and vomiting
"A major incident was declared but this has now been stood down," Secamb added.
Sussex Police urged people to keep their "windows and doors closed for the next few hours due to a chemical incident".
Hastings Borough Council said the "public are not at risk" but waste and recycling services would be affected by the "temporary closure" of the tip.
A suspected chemical incident at a recycling centre in East Sussex left 13 people needing medical treatment.