unless and until the united states+nato and russia are explicitly at war with one another, there is nothing to worry about.

if that happens, it's anyone's guess. traditional models of nuclear deterrence would state that to launch a nuclear strike would be ridiculously irrational, because any state that launches a nuke would prompt worldwide nuclear armageddon and certainly ensure its own destruction. putin probably is a rational actor right now, even if he is prone to tactical error. that said, if he's pressured into a corner...who knows? the chance is extremely slim that he would resort to the button given it is practically suicide, and if he did, there's a good chance others would step in to would stop it (see, e.g., the russian soldier who is credited as averting armageddon during the cuban missile crisis). but it's not impossible. a nuke in the air is most certainly a non-zero chance, even if it is infinitesimally small. a fraction, of a fraction, of a fraction, of a fraction, of a fraction of a percent.

generally, thinking about nuclear warfare is a foreign concept for anyone who grew up after the fall of the ussr because we have never lived under an actual threat of one occurring. the idea of engaging in nuclear war just makes so little sense that it's hard to conceive of any situation where it could happen. but the potential impact of just one error bringing it about is existentially terrifying. it's not the sort of thing I worry about, personally, but it's also not irrational to think about, given that people are, on occasion—and particularly when under stress—irrational.