SDS Studio™Definition: An M-factor is a multiplying factor (1, 10, 100 and higher) given to very toxic Aquatic Acute 1 or Aquatic Chronic 1 substances, which multiplies their weight when classifying a mixture.
With an M-factor of 10, a substance at 2.5 % counts as 25 % in the summation for environmental hazards, so even small amounts can make the mixture H410.
M-factors come from Annex VI where harmonised, otherwise from the substance's toxicity data. SDS section 3 must show them.