Incorrect fact#1: Irkens are allergic to water.
That's a common misconception, Irkens are actually allergic to the pollution in the water. Ya see, the pollution (from smog and stuff) goes up into the atmosphere, then with precipitation, falls back onto the Earth's surface, back into the water supply, and onto the vegetation and crops (cancelling out veggies for dinner) then the livestock consume the crops, tainting the flesh (hence the meat burning to Zim's flesh). It doesn't bother humans too much, but Irkens are way more susceptible to pollutants (probably because of a result to having an eco-clean planet, or fixed their planet because of this. The only things that they can eat on Earth are highly manufactured crops, bleached and powdered, high sugars, and low protein (waffles, pancakes, muffins, doughnuts, etc.).
On one of the show's DVD commentaries, Rob Hummel... I think it was stated this fact, then I used the information on pollution and Irrigation systems.
I have reason to belive that Irkens are actually allergic to beans. They are tough to digest even for humans.