I have kept 1.4 rats in a cement mixing tub rack system for years and never had problems, I also had them in a 20gal when I first started breeding rats (years before I ever thought of ever owning a reptile) and never has repratory problems, I use ossceola pine shavings for horse stalls, which is a bit more dusty then most, but my rodent room also has a couple fans in it to cool it down, the most I can say is the less rats you decide to keep in 1 tank the better, but I personaly have naver had a problem with 1.4 rats in a 20 gal or cement tub.