You should be protected under the ADA if your medical condition substantially limits one or more major life activities (such as lifting, bending, walking, and performing manual tasks); you have a record of such an "impairment" or you are regarded as having such an "impairment." You can let your employer know that you have asthma and request your employer for an accommodation. This should not be regarded as a legal advice, however. Any questions, see an employment in your area. Good luck, and feel free to read a blog on what you should know about your rights under the ADAAA (ADA Amendments Act, which took effect on January 1, 2009).