An assessment should be revisited to ensure that it is kept up to date and an employer should do this regularly. The date of the first review and the length of time between successive reviews will depend on type of risk, the work, and the employers judgement on the likelihood of changes occurring.
The assessment should be reviewed immediately if:
There is any reason to suppose that the original assessment is no longer valid, eg evidence from the results of examining and testing engineering controls, reports from supervisors about defects in control systems; or
Any of the circumstances of the work should change significantly and especially one which may have affected employees exposure to a hazardous substance
The requirement is for a review of the assessment. This does not mean that the whole assessment process will have to be repeated at each review. The first purpose of review is to see if the existing assessment is still suitable and sufficient. If it is, then you do not need to do any more.
If it appears that the assessment is no longer valid, it does not mean that the whole assessment has to be revised. Only those parts that do not reflect the new situation need amending.
Whether or not there is any real change in the situation, there is an absolute requirement to review the situation on a regular basis. Without this, there is a danger that gradual change over a period of time goes unnoticed and the assessment becomes unsuitable and insufficient by default.
For further information about COSHH assessment is available in the [HSE publication A step by step guide to COSHH assessment](http://www.hse.gov.uk/pubns/books/hsg97.htm).