EHO deals with rats and bugs, rubbish, sanitation in restaurants and that sort of thing (works for local council).
EO works for government, looks at larger scale issues such as compliance in recycling, large factories, tips, etc. Also deals with environmental pollution emergencies such as chemical spills, leaks etc and advices police and emergency services. Can be called in to give expert witness in court etc.
EPO might be same as EO, don’t know.
I am not an expert but that’s what I know at the moment.