Wearing street shoes indoors comes at a price: you may be tracking heavy metal dust from roadways, pesticide residues, and other unwanted substances. The Japanese custom of leaving street shoes in a rack by the door and donning alternate footwear is wise, as it reduces what’s tracked in (both visible and invisible soils) and what eventually may become airborne to be inhaled and/or ingested by occupants.

The Carpet and Rug Institute and industry have done studies re: what ends up in carpet; and much of it is tracked in from outside.

See graphic above from the Science of Cleaning report by the Carpet and Rug Institute – Ref. 3.

The abstract, info and links below may be useful.

Abstract

The ability of six different types of contamination control mats currently in use at the entrances to theatre suites and other clean areas to remove bacteria-carrying particles from theatre trolley wheels was compared. Marked differences in the effectiveness of this property were obtained; and all mats showed some disadvantages. Modification of one of the mats has resulted in improved efficiency under working conditions.

Indoor Allergens in Minnesota Schools and Child Care Centers

Scientific American studied the tracking of heavy metals from outdoors to indoors, and a large entry mat reduced levels 6x (Reference Pending).