Yes. It means it's necessary to research this (more).Originally Posted by oldboy
The "what" (noun) you are looking for is "poisoning." The basic idea is:Originally Posted by oldboy
...millions were affected by the poisoning.
(You are correct about the meaning. It means millions were influenced/touched by the poisoning. Even simpler: millions were poisoned.)
The rest of the sentence describes the poisoning. You can think about it like a который clause:
...millions were affected in the poisoning, which the World Health Organisation termed 'the greatest mass poisoning in history'.
(This means: The poisoning was so bad that the World Health Organization termed it (called it) "the greatest mass poisoning in history.")
So the original part ...in what the World Health Organisation termed... is just a way to tell us more about the "what" (noun = poisoning).
Does that help?