My non-expert understanding is that toner is a very fine plastic power. When you make a copy the fuser roller heats it and melts it onto the paper. The image drum in your copier/laser printer is light sensative and the toner is heat sensative, but only at temperatures that would melt it. So I think storage in normal household conditions should be OK. The biggest problem that I see is that the stuff is messy if it spills.