The scientific evidence of potential harm from exposure to methyl iodide is overwhelming. It is heavily linked to miscarriages late in pregnancy, and low survival rates in baby animals exposed in utero and after birth.
Methyl iodide also causes cancer, thyroid disease, and nervous system poisoning. Methyl iodide is considerably more toxic than methyl bromide and also considerably heavier.
It therefore contaminates the ground level air for much longer, rather than dissipating rapidly into the upper atmosphere. Finally, methyl iodide is a very persistent contaminant of both soil and ground water, remaining present in significant amounts for many months.