A bloated milk carton could be totally harmless, or it could be a sign of spoilage. It's totally harmless when it's the result of air pressure changes due to altitude or temperature.

“It’s supposed to be clean when the carton’s assembled or the bottle is filled,” Schneider points out, and if something is amiss there, the carton could bloat and the milk unsafe to drink.

Milk safety in a bloated carton depends on if it is due to air pressure or microbes. If you’re in a high-altitude place and you’re not close to the best-before date, milk should be fine to drink.