As the disease progresses they begin to have symptoms of shortness of breath, cough, and wheezing as their lung function declines. The Journal of Heart and Lung Transplantation published updated guidelines in 2001 for grading the severity of BOS. The original guidelines and classification system were published in 1993 by the International Society for Heart and Lung Transplantation. Their scoring system is based on the changes in FEV1 in patients from their baseline. When patients are first diagnosed with BOS they have their baseline lung function established by doing pulmonary function testing at the time of diagnosis. The BOS scoring system is as follows:
BOS 0: FEV1 > 90% of baseline and FEF25-75 > 75% of baseline
BOS 0-p: FEV1 81-89% of baseline and/or FEF25-75 <= 75% of baseline
BOS 1: FEV1 66-80% of baseline
BOS 2: FEV1 51-65% of baseline
BOS 3: FEV1 50% or less of baseline
The scoring system shows an increased severity of the disease as the BOS number increases. Hematopoietic stem cell transplant
Bronchiolitis obliterans affects up to 5.5% of people who have received HSCT. One of the biggest risk factors after HSCT is the development of GVHD with a 14% risk. Other risk factors post transplant including tobacco use, age of donor, age of recipient, lower baseline FEV1/FVC ratio, non-caucasian race, peripheral and lower circulating IgG levels. Studies have, however, shown mixed results regarding these other risk factors. There has been an association shown between the increased use of peripheral stem cells and the risk of developing bronchiolitis obliterans.
Also, research has shown an increased risk for developing the disease within the first year of transplant if the person is infected with respiratory syncytial virus or parainfluenza virus within the first 100 days post transplant. Inhalants
There are many industrial inhalants that are known to cause various types of bronchiolitis, including bronchiolitis obliterans.Industrial workers who have presented with bronchiolitis:

nylon-flock workers
workers who spray prints onto textiles with polyamide-amine dyes
battery workers who are exposed to thionyl chloride fumes
workers at plants that use or manufacture flavorings such as diacetylDiacetyl is a chemical used to produce the artificial butter flavoring in many foods such as candy and microwave popcorn and occurring naturally in wines. This first came to public attention when eight former employees of the Gilster-Mary Lee popcorn plant in Jasper, Missouri developed bronchiolitis obliterans. Due to this event, bronchiolitis obliterans began to be referred to in the popular media as "popcorn lung" or "popcorn workers lung". It is also referred to as "flavorings-related lung disease". Post-infectious
Typically found in young children and is the most common cause at this age. Generally occurs after a viral infection of adenovirus (types 3, 7, and 21), measles (rubeola), mycoplasma, CMV, influenza, and parainfluenza. Swyer-James syndrome is a rare complication of bronchiolitis obliterans caused by measles or adenovirus. Post-infectious bronchiolitis obliterans is most common in the southern hemisphere particularly in countries such as Brazil, Argentina, Australia, Chile and New Zealand. There was a large prevalence of the disease in these areas during the 1990s and early 2000s.