Why did they use Head & Shoulders to fight the alien invaders in Evolution?

Ah, the selenium!

One of the ingredients in Head & Shoulders is selenium, an element on the periodic table.

The active ingredient in Clinical Strength Head & Shoulders is selenium sulfide 1%

Ignoring the fact that a great number of elements on the periodic table are toxic to humans, the film decided to pick arsenic off of the table and then made this leap of scientific logic:

Humans are carbon-based life-forms, and if arsenic is toxic to us, the "scientists" in the film use the periodic table to decide that the alien life-forms, which are nitrogen-based, must similarly find selenium toxic:

I think the film is ridiculous and haven't seen it in years but this is the one thing I remember about the film.

Here's a reference (and the source of the image) if you want it:

David Duchovny, much by chance and luck discovers that since humans are carbon based and arsenic is deadly to humans, the aliens are nitrogen based and so if you follow the same pattern (2 down 1 over) on the periodic table, the aliens should have a deadly reaction to selenium (which apparently is found in conditioner).

And from the Wikipedia article:

Looking at the positions of nitrogen and carbon on the periodic table, Ira theorizes selenium might be poisonous to the aliens, since they are nitrogen-based, as arsenic is poisonous to Earth's carbon life. Ira's dumbest students Deke and Danny (Ethan Suplee and Michael Ray Bower) recall selenium sulfide is the active ingredient in Head & Shoulders, so the team procures a firetruck and fills it with the shampoo. Ira also gives the Donald brothers an A, much to their excitement.