By its very nature, carpet holds to its surface and within its fibers, that which falls upon it or is ground into it. What is heavier than air; that which we bring in on our shoes, or falls from us and lands on the floor goes into the fibers. Getting it ALL back out is a feat. Most floor installers will tell you that used carpet, even when it looks clean, can weigh up to three times as much as the new carpet which will replace it. Most of the foreign particles are dirt, dust and bearers of things not so good for us to breathe.
Image Dominic Mercier

I agree to David Kurland.
I agree to David Kurland. That is why carpeting and flooring as well should not be overlooked.
I've struggled with asthma
I've struggled with asthma and allergies my whole life. Unfortunately, I'm not a homeowner yet (so I'll have to wait for the bamboo) ... but, as a renter, I refuse to even consider apartments that don't have hardwood floors. I sneeze just looking at old carpet.