I've come across part time health and safety people (and I've done it myself), and a search for part-time health and safety advisor on my favourite jobs board comes up with 3 relevant examples in the UK at the moment.

It is however often the case that if an organisation wants the job to be done part-time, they will get someone to do it alongside other responsibilities.

Peter is right that part-time posts are in the minority but have always been around. I’ve encountered examples in smaller district councils where a lot of their risks have been outsourced, medium-sized upmarket white-collar business-like law firms or finance who want to look after their staff or bespoke/niche SMEs who have a limited range of risks or contacts going on at any one time so don’t need you full time. Oh, and medium sized charities.

 One pitfall is a part-time role may be because there are fewer or less “interesting”  risks to manage which can harm career development. The more serious pitfall is that some employers cut the hours of the role to save money but then you find they still expect the same amount of work as a full-time person did.