The M-factor is a Multiplication-factor which can be applied to substances with aquatic toxicity which have a more severe impact in mixtures than the generic classifications would suggest. The concept originates in GHS and has been adopted in CLP.
As its name implies, the M-factor is a factor, or number. It is applied by multiplying the mass of the substance by the M-factor, before comparing the resulting quantity with the classification limits.
M-factors go up in factors of 10.
There are two sorts of M-factor:
- acute M-factor (applicable to substances classified aquatic acute 1, H400)
- chronic M-factor (applicable to substances classified aquatic chronic 1, H410)
As the M-factor is only for substances which are more hazardous than the worst-case classification, the chronic M-factor does not apply to substances classified as aquatic chronic 2, 3 or 4 (H411, H412 or H413 respectively).
A substance classified as both acute and chronic aquatic toxicity, category 1 (H400 and H410), this means it can have two M-factors.
M-factors are published with the Harmonised Classifications https://ttenvironmenta.wpengine.com/clp-knowledgebase/harmonised-classifications-under-clp/. They can also be published in REACH dossiers and the Classification and Labelling Inventory.
Theoretically, it could be possible for environmental hazards to have Specific Concentration Limits, but in practice this does not occur, and M-factors are used instead.
Where an M-factor has not been published, but a substance in a mixture is classified as aquatic acute 1, H400, and/or aquatic chronic 1, H410, you can derive your own M-factor(s) from data published in the REACH dossier, or in an SDS, or from another reliable source.
M-factors are based on the lowest LC50/EC50 concentration in any one of the three trophic (feeding) levels in the aquatic environment. For details on how to derive an M-factor, see: https://ttenvironmenta.wpengine.com/clp-knowledgebase/environmental-hazard-classification-of-mixtures/ .