Percent of kitchen area samples within different CO concentration ranges by minute of a day for the intervention group (B,D,F) and control group (A,C,E) at baseline (A,B), at follow-up samples taken at the 3-, 6-, 12-month time points (C,D) and at 18-, and 24-month time points (E,F) in rural Peru. Samples were replaced by : for direct reading CO measurements. The sample size for baseline, 3-, 6-, 12, 18-, and 24-month visits for control and intervention households are: and 85, and 80, and 82, and 77, and 79, and 79. During the second year (follow-up visits:18- and 24 months) the control participants received the LPG stove intervention and vouchers for 1-y supply of free fuel and intervention participants stopped receiving free fuel but kept the LPG stove. Note: CO, carbon monoxide; LOD, limit of detection; LPG, liquefied petroleum gas.