Shelly Kagan (2011, "Do I Make a Difference?"):
a single molecule of the toxin makes no difference to anyone's health. To be sure, if enough molecules are taken in, the result is sickness or death; but one molecule, more or less, simply doesn't make any difference at all to anyone's health.
Imagine, next, that there are thousands, or tens of thousands, of similarly polluting factories around the nation (or the world). Each scatters its toxins so widely that no single individual ever takes in more than a single molecule from any single plant. But because there are indeed thousands of such factories, many people do take in enough of the toxin to become ill. ... But for all that, it seems as though each factory owner can truthfully say to himself that it makes no difference whether or not he pollutes, for his decision puts at most one extra molecule of toxin in any given individual, and by hypothesis a single molecule, more or less, simply doesn't make a difference to anyone's health. When I think about my own decision whether or not to pollute, then, I have to admit that my polluting doesn't actually harm anyone, since it doesn't make a difference to anyone's health.
Is the above reasoning correct? If not, is there a name for the error or fallacy committed here?