The issue there, too, is of chemicals breaking down under conditions of light and heat. The by-product in the case of aspartame is said to be formaldehyde.
I mostly don't drink soda, but I went through a period of drinking diet pepsi. But after a few days, I started having restless legs and other neurological symptoms. I ended up going off the soda and the symptoms disappeared. Later on, I thought it was probably just a coincidence, so I bought some diet drinks again. Same symptoms returned. So I threw the stuff away and have been fine since. Regular soda doesn't bother me though . . . except for the calories. . .
But the FDA is certain aspartame is safe. I don't know why the FDA wouldn't be as worried about formaldehyde in our drinks as benzene. Here's the other point of view:
http://allergies.about.com/cs/aspartame/a/bluc_mgold.htm"The formaldehyde exposure from aspartame is significant. Aspartame breaks down into methanol, amino acids and several other chemicals. The methanol is quickly absorbed and converted into formaldehyde. The methanol found in foods and alcoholic beverages is also absorbed, but there are "protective chemicals" in these traditionally-ingested foods and beverages that prevent the conversion of methanol to formaldehyde.
Formaldehyde is known to cause gradual damage to the nervous system, the immune system and has recently been shown to cause irreversible genetic damage at long-term, low-level exposure.
The most recent, independent research shows that the situation related to aspartame may be more serious than simply regular formaldehyde exposure. This research on animals demonstrates that the formaldehyde accumulates as adducts (bound to protein molecules) in the organs (brian, liver, kidneys) and tissues of the animals when aspartame is ingested at relatively low doses"
"These are indeed extremely high levels for adducts of formaldehyde, a substance responsible for chronic deleterious effects that has also been considered carcinogenic. .... "It is concluded that aspartame consumption may constitute a hazard because of its contribution to the formation of formaldehyde adducts."