What is the difference between the old EN149 standard and the EN149: 2001 standard?

Simply, the new standard combines solid and liquid test methods for each respirator, but precisely:

Under the old EN149 standard the classifications were FFP1S, FFP2S, FFP2SL, FFP3S and FFP3SL. These have now been superseded by just FFP1, FFP2 and FFP3.

Previously, whilst the old 'S' type respirator protected only against solid and non-volatile liquid particles, the 'SL' type had also to protect against oil based mists. This could potentially lead users to select by mistake an 'S' model instead of the correct 'SL' model when one or more of the hazards was an oil based mist.

Today with the new standard in place all models conforming to EN149:2001 must offer protection against all of the hazards mentioned previously, which can only decrease the risk to the health and safety of the end user.