Way too many unknowns to answer or to give advice, especially taking into account possible litigations and so on. In general, and with every possible disclaimer about how this is not an advice and that you should seek professionals capable of assessing the individual situation correctly and not ask random forum users: in a typical setup (ie garage is well ventilated, not used to store commercial amounts of solvents/fuels/whatever car chemicals, freezer is opened just to put something in or take something out, and used for food only) I wouldn't care much.
But if it is going to make you feel better and safer design some kind of a procedural routine around taking things out/closing them and/or keeping them in sealed bags. It won't change much in terms of risk of contamination (which is already minute) but it will make you feel safer.