Randomhead’s answer is correct: “it” is ”the nuclear industry”. But you’re right to wonder whether “data” was meant. In theory, “long-term exposure” and “cancer” are also possibilities.
I wanted to add some more detail—some extra guidance on working out what pronouns like “it” refer to. I was going to make a comment on that answer, but then I kept coming up with more variations… and more… and more…
So now you get this long answer instead!
Agreement
As randomhead says, “it” could not be “power plants” or “authorities”, because “it” is singular. Pronouns should[1] agree in number with what they refer to.
“Data” is an odd case though: it’s technically plural, but it’s usually construed as singular.
Is the data available yet?
Are the data available yet?
I would expect to see the former, except in formal writing. In speech, only a very pedantic speaker would use the latter, and they’d get some funny looks!
Something else
“It” could not be “authorities” for another reason: they’re part of the same noun phrase:
the authorities responsible for monitoring it
Even if the sentence used a singular “authority”, “it” still couldn’t mean the authority for this reason. The pronoun has to be[2] something other than the thing we’re currently describing, or else a reflexive pronoun (“itself”) would be used.
Something that makes sense in context
I read a sentence recently—I don’t know where[3]—that was given as an example of something that humans can understand fine, but computers (using natural language processing software) struggle with. It went something like this:
I couldn’t fit the box in my suitcase because it was too large.
Syntactically, “it” could equally refer to “the box” or “my suitcase”. But logically, “it” should be “the box”. If the item is too large for the container, it (the item) won’t fit.[4]
Issues like this are very common when reporting interactions between people, and often it’s not possible to be sure which person the pronoun refers to. For example:
Jack told Michael that he had made a mistake.
Who made the mistake, Jack or Michael? We can’t be sure! Maybe we can guess from context. The speaker could have repeated the noun (“Jack told Michael that Jack…”) instead of using a pronoun, which may be awkward, but it’s clear. Sometimes writers will clarify this like so:
Jack told Michael that he, Jack, had made a mistake.
Something recently mentioned
Randomhead’s suggestion was to find the nearest preceding noun (phrase). This is a good guideline, although it’s not strictly followed—see the examples above.
Sometimes pronouns are just plain ambiguous, and even native speakers will struggle to work out what is meant. This is a sign of poor writing that should be clarified.
…or not mentioned (yet)
Sometimes a pronoun refers to something that hasn’t even been mentioned. We need context to identify the referent.
Do you like it?
Or a pronoun can refer to something that isn’t mentioned until later (the linguistics term is cataphora).
It’s a strange sensation, the feeling you get when you hit your funny bone.
She has money. She has fame. She has romance. But Leah is still unhappy.
Note that these aren’t the same as dummy pronouns, which don’t really refer to anything at all.[5]
It’s raining again.
[1]: This rule is sometimes violated in informal or dialectal speech, but that isn’t what we’re dealing with here.
[2]: I suppose it’s possible to have a long noun phrase that’s so convoluted that “it” refers to part of the same noun phrase. But I can’t think of any examples, and it would be very bad style.
[3]: I will try and remember to come back and edit this if I find it again. Of course, if anyone reading knows where it’s from, please do point it out!
[4]: Hypothetically, you could have a problem where the container is too large, and the item has too much room to move around. But then “fit” wouldn’t be the right verb, for one thing. Maybe you could say “put” instead of “fit”. Even so I would expect the audience to think the box was the one that was too large, and the speaker would need to clarify what they meant.
[5]: Or, depending on your interpretation, they may refer to “a general state of affairs”, as that Wikipedia article discusses.
