Slippers never go outside. If they do its for but a few seconds to avoid picking up a ton of dirt.

What people seem to miss when thinking about how dirty their shoes are are the chemical residues from vehicles. Exhaust pipes drip contaminated water, oil drips from cars, brake dust accumulates in those wet and oily spots on the road, etc… Every time you cross the street or walk in dense public areas your shoes pick that stuff up. If you walk around your house with those same shoes, you invite toxic residues into your living space. The effect might be negligible but the thought makes me wanna gag.