The aircraft, a Boeing 737-800, carried 155 passengers and six crew members - though despite the drama, none of them needed medical care once they deplaned at Washington’s Dulles International Airport.
In a statement to the New York Post, United Airlines said: “The aircraft is currently undergoing a deep clean and we are working to get customers on their way to Boston soon”.
A biohazard incident usually refers to being exposed to a biological substance, such as vomit or faeces, which could be bad for a person’s health.
Last year, another biohazard event forced Delta Airways to divert just an hour and a half into a flight from Georgia in the US to Barcelona in Spain.
“We’ve had a passenger who’s had diarrhoea all the way through the airplane, so they want us to come back to Atlanta,” the captain of the A350 was recorded saying.
One passenger took to social media to express disgust: “It was pretty bad,” they wrote on Reddit. “It [had] dribbled down the aisle, [and] smelled horrible.”
Passengers spent the entirety of the eight hours and 13 minutes in the same plane, with lingering smells.