You might notice the M61 filter’s time patches turning light blue on your unit’s M50/M51 protective masks. WP 0004-00 in TM 3-4240-542-13&P (May 08) tells you that if the center of the patch turns blue, then the M61 filter is expired and must be replaced.
A white time patch means the filter is serviceable. But any shade of blue means the filter’s expired and needs replacing. So don’t use it in chemical or biological environments, including the CS gas chamber, if the patch is any shade of blue. That’s because the particulate filter may be wet, which won’t protect you from CS gas.