I'm not sure how you got from brake cleaner to solder but yes, the vapour that comes off hot flux is indeed toxic (I remember all the warnings at work when they realised

). I got this off a bike forum, further down someone mentioned chain cleaner was more or less the same.
Used engine oil is carcinogenic and can cause dermatitis (personal experience), you wouldn't want to get brake fluid in your eyes, don't drink antifreeze etc. etc. There is stuff to be worried about but in these days of totally stupid paranoia, where you need to carry out a risk assessment before getting out of bed in the morning and CORGI accreditation before opening a window, it's very easy to get overloaded with it all.