Table 1.

Inclusion criteria for studies

Criteria
1	Study included COPD patients as a comparator group or a population defined by lung function
2	Participants must be free of malignancies
3	Articles must be original research and have been published as a full-text in a peer-reviewed journal
4	Studies must have been carried out in humans (exclusion of in vivo and in vitro study designs)
5	Studies must assess relative levels of DNA methylation
6	Analysis of DNA methylation must have been in peripheral blood
7	Abstracts must be available in English.