DP: Not quite sure where you're coming from on this one - but I'm fairly sure (stand to be corrected) that RIDDOR does not explicitly mention Asbestos... However "The unintentional release or escape of any substance which could cause personal injury to any person other than through the combustion of flammable liquids or gases. " is one of the scheduled 'dangerous occurrences' (not offshore).... So I will leave it to others to argue whether any release of Asbestos fibres is always reportable. (Very poor - awful - drafting of legal standards as is perhaps to be expected these days)

But there are other regulations which require some things to be reported to HSE (NOT under RIDDOR)... They include things like LOLER examinations that identify serious defects. And others... If this is where the query is directed, then you may need to re-think your assumptions?

From the HSE website:
2. Incidents occurring at any place other than an offshore workplace
Hazardous escapes of substances
27 The unintentional release or escape of any substance which could cause personal injury to any person other than through the combustion of flammable liquids or gases.

The substances covered by this definition may be in any form: liquid, solid (eg powder), gaseous or vapour and may include, eg:
substances which may be hazardous to health (eg asbestos, phosgene, toluene diisocyanate)
substances which may be either corrosive or potentially hazardous by virtue of their temperature or pressure (eg nitric acid, molten metal, liquid nitrogen)

This definition includes incidents which present a fire or explosion hazard (eg combustible powders), but not in relation to releases of flammable liquids or gases, where the relevant thresholds in 26 [link to paragraph 26] are not exceeded.

Examples of the kinds of incident covered by the definition are escapes arising from the failure or breakage of plant, pipes, equipment or apparatus; failures of process control; the operation of a relief valve or bursting disc where the escaping substance is not safely controlled or directed, and spillages from containers and equipment.

Releases from plant etc during the normal course of operation or maintenance (eg during sampling, packaging or draining of lines) that are sufficiently well controlled to ensure that no person is put at risk would not be reportable.

In some cases, the decision as to whether or not an incident is reportable will be straightforward, eg if a person is exposed to a hazardous substance at a level which exceeds established safe limits (eg a Workplace Exposure Limit).

However, most incidents require judgement. Various factors are relevant including: the nature of the substance and its chemical, physical and toxicological properties, the amount which escaped and its dispersal, and whether people were, or could foreseeably have been, exposed to a significant risk as a consequence of the escape.
http://www.hse.gov.uk/ri...dangerous-occurences.htm