Cigarette smoking reduces DNA methylation levels at multiple genomic loci but the effect is partially reversible upon cessation.

Tsaprouni LG, Yang TP, Bell J, Dick KJ, Kanoni S, Nisbet J, Viñuela A, Grundberg E, Nelson CP, Meduri E, Buil A, Cambien F, Hengstenberg C, Erdmann J, Schunkert H, Goodall AH, Ouwehand WH, Dermitzakis E, Spector TD, Samani NJ, Deloukas P.

Epigenetics. 2014 Oct;9(10):1382-96. doi: 10.4161/15592294.2014.969637.

http://www.ncbi.nlm.nih.gov/pubmed/25424692