Participants will be asked to come to the Clinical Research Center (CRC) for a visit lasting about one hour. Participants will be asked to review and sign an informed consent document to participate in this trial. If participants agree to participate and sign the consent, the following procedures will be done. If they have not had pulmonary function testing performed in the past 18 months, standard spirometry testing will be performed. Participants will perform two six-minute walk tests, one indoors and one outdoors. First, they will be randomly assigned to perform either the indoor or outdoor testing first. Then participants will have their vital signs taken, perform the first six-minute walk study and immediately after have their vital signs taken again. Participants will have a rest period between the two six-minute walk tests of at least 30 minutes. Participants will then perform the second six-minute walk test with the same above procedures for the first test.