In low enough concentrations Roundup is not all that toxic to humans and usually breathing in just a little bit of the Roundup spray is not gonna harm you.
However if you do breath is a lot of the Roundup then it could be toxic and harmful and also if you get too much Roundup on your skin and do not wash it off quickly enough it could lead to health problems.
But if you get just a little bit of the Roundup on your skin accidentally you should be perfectly okay as long as you wash it off as soon as possible.
I've used Roundup a lot without any problems but it's best to wear a breathing mask and long rubber gloves as well.