Medical Associations:

American Thoracic Society - http://www.thoracic.org/
American Academy of Allergy, Asthma and Immunology -http://www.aaaai.org/home.aspx
American College of Allery, Asthma and Immunology - http://acaai.org/
American College of Chest Physicians - http://www.chestnet.org/
American Academy of Pediatrics - https://www.aap.org/en-us/Pages/Default.aspx
American Association of Respiratory Care - https://www.aarc.org/
Association of Asthma Educators - http://www.asthmaeducators.org/

Other links:

2005 ATS/ERS Spirometry Official Statement – Download Here
Global Lung Initiative - http://www.spirxpert.com/gli_intro.html
GLI Reference Values - https://vitalograph.com/resources/gli-normal-values
The Global Initiative for Chronic Obstructive Lung Disease (GOLD) - http://www.goldcopd.com/