For reasons I’ve never fathomed in 6 years of living here, there are very often ruptured (likely run over by car) AA/AAA batteries in my neighborhood and surrounding area parking lots. I can’t figure out why batteries, of all objects, are being left out on the parking lots so often and getting run over by cars, but they are. Often the batteries are already split open, with contents leaking out. Sometimes I almost wonder if this is purposeful littering; batteries aren’t exactly objects you’d scatter about without noticing.
Anyhow, how toxic are the contents really? AFAIK they are often alkaline and/or nickel-cadmium batteries, and if run over by cars repeatedly when already ruptured, would presumably be having their contents spread about by the car tires, people walking on or around them, etc.
Don’t eat that stuff or put it in your eye, but otherwise, you don’t need to panic over incidental exposure. Potassium hydroxide (the alkaline inside) can be very caustic but the small amount in a dry cell is very little to worry about should it be cleaned off.
Also, if it’s been out in the elements for a while, the potassium hydroxide will oxidize to potassium carbonate, which is relatively safe (it’s even used in some foods). If you see a salty looking white powder, it’s most likely already reached this state.
ETA: older dry cells (like decades old) did used to contain a fair amount of mercury, which is something to worry about, but anything you’d buy these days shouldn’t be a concern.
I’m one of those guys who picks up garbage as my chosen form of community service. I see all sorts of garbage, including batteries. You may notice them more because they don’t blow away like a lot of other trash. The most common types of trash I encounter are cigarette butts (grrr!), cans/bottles/cups (the bastards buy their fair trade coffee and litter their cups/lids/straws), and junk/fast food wrappers/containers.
It’s the rechargable ones you have to worry about.
NiCds contain cadmium (duh!), which is Not A Good Thing.
NiMhs contain a really nasty electrolyte that can off gas toxic stuff it if gets punctured in certain situations. The electrolyte is also very bad if you gets on your soft tissue: skin, eyes, mouth, etc. Basically almost all of your exposed body.
LIons vary a lot in composition so the particular ways it could hurt you also vary a little bit. But none of these ways are good.
Both. NiMH used to require a special charger to avoid overcharging. You can buy smart chargers now that will handle the different battery chemistries appropriately.