A chemical plant spill caused a “noxious cloud” to form over northeastern Kansas, forcing the evacuation of the factory and a school.

At least 34 people were treated at Atchison Hospital for respiratory discomfort on Friday, authorities said.

Smoke was reported from the MGP Ingredients plant in Atchison, posing a breathing threat, according to Atchison County emergency manager Wes Lanter.

Two chemicals were inadvertently mixed within the sewage facility at the plant, creating a gaseous plume, said city manager Trey Cocking.

The St Joseph News Press reported that one school was evacuated and several classes kept students and staff indoors as a thick fog spread over the town, which smelt like chlorine.

"This was a large plume that enveloped a large area. We’re very fortunate this time that there were not more injuries," Atchison fire chief Ted Graf said.

The Kansas Department of Transportation closed three nearby highways until 11am.

The Environmental Protection Agency said it was sending investigators to Atchison.

Water and foam was applied to dissipate the plume of chemicals.

The incident was reported around 8am on Friday morning and an all-clear was given by 10.40am.

MGP Ingredients supplies premium distilled spirits and speciality wheat proteins and starches.