Normally, I enjoy living on the higher floors of buildings. I get a better view. If the buildings are insulated the way they are in Korea, it saves me on heating costs. I rarely turn on my Ondol (floor heating system), but I am warm because of the heat that rises from lower floors.
On the other hand, the people that built my apartment building did not consider tight tolerances for gaps to be particularly important. Theoretically that would mean I could get fresh air, but lots of Korean people smoke. I wake up at night and feel like crap because of all the smoke that goes up the ventilation shafts up to my apartment. Hence I had a goal to seal the gaps that are responsible for this problem.
To my knowledge, Korea does not have a DIY store like Homebase, HomeBox, Home Depot, Lowes, etc. Lowes even has a Korean version of their website, presumably for the large population of Korean people living in Los Angeles. However, I was able to find some silicone sealant to seal some gaps in the loo. For other places, I used my collection of plastic bags and tape to seal off openings that need to be opened, but do not need to be opened very often. I still need to seal the W.C. fan vent (which blows into the apartment and not out), but the changes I made have already made a noticeable improvement in air quality.