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Background: A portable breath carbon monoxide (CO) monitor has a high cross-sensitivity to hydrogen (H 2 ). This study examined the influences of H 2 after consuming milk on the detected CO values using three types of portable CO monitors., Materials and Methods: Exhaled breath from seven participants (four healthy nonsmokers and three smokers with otherwise unknown comorbidities) was collected in sampling bags. The participants then consumed 200 mL of milk, and the exhaled breath of each was collected in separate bags every 30 minutes until 9 hours later. CO and H 2 in the bag were measured using a gas chromatograph as a reference analyzer, and CO was also measured using three types of portable CO monitors., Results: After consuming milk, H 2 levels were significantly higher, and CO levels were not significantly elevated as measured by the reference analyzer. However, CO levels in monitors A and B were significantly elevated, even though participants did not smoke. The H 2 levels in the reference analyzer significantly increased and reached a maximum 4.5 hours after consuming milk. The difference in CO levels between the reference analyzer and each monitor increased significantly after 5 or 5.5 hours., Conclusions: This study suggested that the breath CO monitors with a cross-sensitivity to H 2 responded to H 2 as CO in the exhaled gas and measured higher than actual values after milk consumption. The extent of the influence of H 2 differed depending on the type of CO monitor. It is necessary to consider milk consumption when assessing the smoking status of people using portable CO monitors., Competing Interests: The authors declare that there is no conflict of interest regarding the publication of this article., (Copyright © 2022 Kiho Miyoshi et al.)