TET1 encodes a dioxygenase that consecutively converts 5‐methylcytosine (5 mC) into 5‐hydroxymethylcytosine (5hmC), 5‐formylcytosine (5 fC) and 5‐carboxylcytosine (5caC), thus playing a key role in active DNA demethylation and resulting in transcriptional activation of downstream genes such as VEGFA, which is known to be associated with lung function,52 particulate matter exposure and asthma.45, 53, 54, 55. This evidence concerns the gene TET1 and asthma.