I have been working with these masks for 15 years and I have never come across a definitive answer to this question. If you think about there is a world of difference between a mask that is used once a week for ten minutes and one that is used all day long in some dusty atmosphere. No manufacturer will tell you whether masks need changing. My rules of thumb are:

1. If the filter looks dirty or is wet change it immediately
2. If you wear it all day in a dirty environment change it at the end of the shift
3.  A couple of hours a day every day change at the end of the week
4.  Occasional use change every month or so

This is very rough and ready and might be completely wrong for you. Someone might have a more scientific way of measuring the effectiveness of masks. If they have please tell me about it.

Hi Lawlee

Check out HSG 53 on Respiratory Protection at work, particularly at appendix 2

It's a good guide

Regards

Mike

Just a quick point to note, this really depends on what mask and filter you are talking about, I think we have inferred P3 filters attached to re-useable masks, however other filters such as AX for example have a strict single use rule.

A2 / ABEK or other combinations may also depend on factors such as odor threshold Vs harmful effect level, for example an A2 mask protecting against Xylene may be used up until a point where "breakthrough" is detected by the user (obviously some exceptions apply to these rules, anosmic etc...).

The issue with continued use of paper filters I recall is due to the increased resistance for air to pass through increasing the risk of leakage - for face sealing masks - whereby a P3 filter in a PAPR type mask could operate until the unit could no longer meet the airflow / pressure requirements due to the extend of filter loading / blockage