The "threads" are actually a plastic, I believe, so not germ-holding cotton, and they are completely internal. When you check for placement - you are actually getting very personal with yourself (too not get too graphic here!). So no, smell is not an issue. I am on my second Mirena, and admittedly have not checked on mine in a very long time. It really is a fit it and forget it kind of birth control. I had trouble with mine in the beginning - my body does not like extra hormones, so spotted and had irregular periods for a looong time - almost had it taken out a couple of times but my doc convinced me to stick it out. So glad I did! Now I have virtually no period and my very fertile body has remained without a new resident for almost 7 years. I thought about switching to the copper IUD that lasts 10 years and has no hormones, but my doc said I could expect worse periods - possibly worse than before I went on Mirena - no thank you! I'll stick with what works so far!