Rank: Super forum user 
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What is the acceptable load for legionella in a water system? Is there an action level or is there zero tolerance for legionella in water systems?
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Rank: Forum user
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Hi My understanding is that there is not a tolerable/acceptable level of legionella in water systems. However, there is a hierarchy of recommended actions depending on the levels. Table 2.2 of the HSG274 Part 2 sets out the recommended actions http://www.hse.gov.uk/pubns/priced/hsg274part2.pdf Sam
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Rank: Forum user
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There is a tolerance on sampling. If a high CFU is measured then an additional control may be needed. For example a sample taken from a Safety Shower showing a high level of CFU can be controlled by a C&D. A further sample would then be taken. You may decide just to flush before the 2nd sample but a simple C&D normally does the trick.
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Rank: Super forum user 
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Indeed, there is a tolerance. In some systems you simply might have to accept a low level of microbiological activity, including legionella due to unshiftable colonisation. Hardly desireable but maybe nothing that can reasonably be done.
Or a result may be a one-off and not repeatable - trend is just as important as individual figures because labs do make mistakes too.
So the table SamJen mentions is appropriate as a guide which implies <100cfu/l is generally acceptable (others may disagree) but that still needs to be considered within a risk assessment framework. For example, in healthcare premises a stronger/larger reaction to a low level figure might be entirely appropriate.
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