If you have staff just walking around your workplace, you need to make it clear where they can and cannot be. One of the best things you could do is invest in some construction site signage. This can be effective as you ensure that areas with specific access needs – such as wearing PPE – are clearly marked and labeled.
Staff will always be keen to find a way to blame the business for its own mistakes. It’s sadly just part of being a business owner that you need to find a way to work around that. My advice is quite simple – make sure that you buy a clear sign that gives your staff absolutely no chance of saying they ‘did not know’ or feigning ignorance of the need for PPE.