Dust
You will have noticed that there is lots of dust on the streets, unless you live in Finland, where the streets are immaculate.
You will also have noticed that on dry days soily ground is dusty. It is estimated that 35% of household dust originates in outdoor soil.
Naturally, as much of it originates from the ground, dust contains all the sort of things that are on the ground, such as pesticides, weed killer and lead. Things which are not good for your health. Keeping as much of this dust out of the home is a really sensible idea and this means taking off your shoes at the door and asking visitors to do the same.
Even if the dust that gets in is not full of toxins, it is good to reduce it. It reduces the quality of indoor air and can be a source of allergies.
A fashionable strategy is to remove carpets, as they absorb dust. However, this may be counterproductive as without the carpet, the dust is exposed. If you are going to go carpetless, you either need to sweep very often. Hence, whether you choose to opt for carpet or sans carpet, a shoes-off policy is totally adviseable.