There have been some studies (I have no links right now because I'm just posting) that show that a little poison can sometimes be better than none. Some homeopathy uses that philosophy. So, four cigarettes per day are better than none. A little radiation, mercury, etc., is better than none.
Also, just observationally, I see a lot of posts here of people who had no problems with wheat or dairy, etc., until they cut it completely from their diets. Then one exposure, and they're sick. In addition, there are some people who think that one of the reasons we get sick so easily is that we have sterilized our environment to the point of paranoia.
You can probably see where I'm going. Just for me, I think a little exposure to the toxins in the environment is better than none, and certainly better than chronic exposure. So, the occasional bagel won't kill you, but cereal for breakfast, a sandwich for lunch, and pasta for supper for 30 years, won't be good. A good cigar once a month won't kill you, but 30 cigarettes per day probably will.
Oops, how I do go on. I don't worry about radioactive crap because I don't eat enough nori for it to be a concern. Now, if I start eating 50 sheets per week, I'll take a second look. Because sometimes the dosage determines whether or not a thing is a poison.