Bear spray expiry dates debunked: capsaicin stable, weight check saves cans

A LessWrong post by Bruce Middleton challenges conventional wisdom on bear spray expiry dates. After noticing his 4-year-old can was still full, he researched the science. The active ingredient capsaicin does not degrade, and propellant leakage can be checked by weighing the can (discard only if weight drops below 75% of original). His own test fire (0.5 seconds) consumed 7% of net weight, consistent with label specs, meaning he can test 5 times before hitting the discard limit.

Middleton also eviscerates a 2020 Journal of Wildlife Management study from Brigham Young University that claimed pressure drops logarithmically and recommended against test-firing. The researchers used a tiny 24ml collection tube, causing pressure buildup that prevented propellant from flashing to gas. Real aerosol physics: propellant remains liquid at ~3.5 bar, and head pressure stays constant until liquid is gone. The study's flawed methodology produced 'comical pseudoscience.' The takeaway: your bear spray likely lasts far beyond its printed expiry date, and annual test fires are safe.