Two city buses were spewed with a noxious substance in separate incidents in less than a day this week.

The first instance occurred on a B15 bus that runs between JFK Airport in Queens and Bedford-Stuyvesant in Brooklyn. It happened just before 5 p.m. Thursday, when a rider sprayed an unknown substance into a crowd of passengers. The driver and passengers fled the bus near the corner of East 98th Street and Blake Avenue in Brooklyn. Cops found the spray canister but said they haven’t yet figured out what’s in it.

Then about 4:30 a.m. Friday, an angry rider who failed to pay when he got on the bus became enraged when the driver couldn’t make a stop at Rockaway and Hegeman avenues in Brooklyn because of construction. The man pulled out a giant can and sprayed it into the partition protecting the driver.

“The fumes from it were choking me and burning my eyes,” said the driver, who asked not to be named. “I had to jump off the bus.”

The driver and some of the passengers were taken to Brookdale Hospital.

Cops said they are not sure if both instances involved the same perpetrator.

The head of the city’s transit union said the incidents show how perilous an MTA worker’s job can be.

“These are despicable acts,” said Transport Workers Union Local 100 President Tony Utano. “This is exactly why the union fought so hard to get the MTA to install partitions on buses. Bus operators never know who is boarding or what they might do.”