https://chemicalwatch.com/69212/no-deal-brexit-could-make-uk-chemicals-dumping-ground
Chemical watch is an online publishing outfit formed in 2007. They are simply scaremongering. It is the season of goodwill so Greenpeace need to go out and get their begging tins out.

You make a mistake holding up the EU as the goldstandard for chemicals and their control, distribution and removal

International treaties have existed for decades: A link somewhat out of date but gives the basics.
https://archive.epa.gov/oswer/international/web/html/200610-international-chemical-hazards.html#multilateral
Having worked internationally using various chemicals I hsve seen a progressive tightening of the regulations over time concurrent with the growth of the role of safety officers to police their implementation.
http://www.engineersjournal.ie/2016/09/06/ghs-classifying-chemical-substances/
I have also worked on landfill hazardous waste cell construction and contaminated sites. From my own experience I know that over time regulations become ever more stringent. The international aspect of many regulations makes the thought of their relaxation after brexit risible.

http://www.legislation.gov.uk/uksi/2005/894/contents/made
In a nutshell many of the regulations are subject to International treaty, EU regulations are a subset.
To suggest that brexit will lead to relaxation of legislation in my view is ludicrous. It takes a couple of minutes on the internet to determine the hazmat classification and hence handling instructions for any chemical. Who in their right mind would volunteer to ignore them?