Milk often was preserved with formaldehyde about a century or two ago. You would think that people nowadays would know better. Well, maybe they do, but maybe they also aren't aware of ways that other things can be converted into formaldehyde. Interestingly enough, if you are a diet soda drinker, you may be doing just that: drinking formadehyde and not even knowing it.
Hopefully, you are keeping your soda in the refrigerator. Hopefully, you are never storing it in a hot pantry or car, to drink it warm. Not only does that taste absolutely gross, but that's when you're drinking formaldehyde. You see, 10% of aspartame is methanol. If that methanol is heated to over 86 degrees, it turns into formaldehyde, which will deposit itself into the fat cells, usually to the hips and thighs. Diet Soda also has been shown to actually make people fatter (who would've thunk). So, as you're getting fatter, you're also storing more and more formaldehyde into your system. Though you might be preserving your body, what good is that if you won't be around to enjoy it (because of obesity, diabetes, heart disease)?