Impact of Measuring Distance and Exposure to Cold Outdoor Environment on the Temperature Measurement Using a Non-Contact Infrared Thermometer
Fever detection plays a critical role in the screening and control of highly contagious febrile diseases. While axilla or rectal temperatures are considered the gold standard for fever detection in clinical practice, they are impractical for mass screening. Rapid, cost-effective, and non-contact temperature measurement tools with high sensitivity and accuracy are essential. Non-contact infrared thermometers (NCITs) have become one of the most commonly used tools for screening coronavirus disease 2019 (COVID-19) in public settings in China, as fever is a typical and early manifestation of the disease. However, the accuracy of NCITs can be influenced by factors such as the distance between the sensor and the skin surface, as well as exposure to cold outdoor environments. This study aimed to assess the impact of these factors on temperature measurements using an NCIT.
The study was conducted in a controlled indoor environment with a stable temperature of about 20°C, while the outdoor temperature was approximately 10°C. A total of 30 healthy participants aged between 18 and 35 years were enrolled, and 29 completed the experiment. Participants were excluded if they were pregnant or breastfeeding, had engaged in vigorous exercise before the study, or had thick bangs that could interfere with temperature measurements. The NCIT used in the study was the Berrcom Non-Contact Digital Thermometer JXB-180.
Participants first sat indoors for 45 minutes to acclimate to the indoor temperature. Forehead and wrist temperature readings were then taken from three different distances: direct contact (Close), 3–5 cm (Standard), and 10 cm (Far). Afterward, participants spent 15 minutes outdoors, during which they were instructed to avoid strenuous exercise. Upon returning indoors, forehead and wrist temperatures were measured every 3 minutes for 30 minutes from the standard distance. Participants were allowed to adjust their clothing according to personal preference, and wrist temperature measurements were taken from a site covered by clothes.
The recommended measuring distance in the NCIT manual is 3–5 cm, which was set as the “Standard” distance. Measurements taken at the Close and Far distances were paired with the Standard measurements for Bland–Altman analysis using SPSS 26.0. A difference of no more than 0.2°C between the Far or Close measurements and the Standard was considered clinically acceptable, given the NCIT’s accuracy of ±0.2°C. The temperature taken 45 minutes after the study began was considered the baseline. After 15 minutes outdoors, the body temperature at each time point minus the baseline was used to generate heatmaps using the pheatmap package in R version 3.6.2.
When the NCIT displayed “Low” for temperatures below 36.0°C, these values were set to 35.9°C for analysis. The time point when the temperature reading was no more than 0.2°C lower than the baseline was considered the moment when the body surface achieved complete rewarming. The mean rewarming time and the upper limit of the 95% confidence interval were calculated. If no obvious pattern was observed, temperature ranges were calculated to indicate the extent of temperature fluctuation.
The results showed minimal impact of measuring distance on NCIT readings. The mean differences between Standard and Close measurements for the forehead were 0°C, while the differences between Standard and Far measurements were 0.0690°C. For the wrist, the mean differences between Standard and Close measurements were -0.0862°C, and between Standard and Far measurements were 0.0414°C. The 95% limits of agreement (LoA) for these measurements were (-0.1482°C, 0.1482°C), (-0.0706°C, 0.2086°C), (-0.2730°C, 0.1006°C), and (-0.1512°C, 0.2340°C), respectively. Bland–Altman diagrams showed that only 2, 1, 2, and 1 out of 29 data points were outside the 95% LoA for the respective measurements. Within the 95% LoA, the maximum differences between each pair were 0.1°C, 0.2°C, 0.2°C, and 0.2°C, indicating that temperatures measured at Close or Far distances were in acceptable agreement with the Standard approach.
Forehead temperature readings were more consistent than wrist readings. After 15 minutes outdoors, all forehead measurements fell below the baseline, but 90% returned to baseline within 9 minutes. The average rewarming time was 6.2 minutes (95% CI 5.0–7.5 minutes). By the end of the study, 21 participants had forehead temperatures similar to the baseline (no greater than 0.2°C from the baseline), while 7 had higher temperatures, with increases of 0.3°C in 3 participants, 0.4°C in 1 participant, 0.5°C in 1 participant, and 0.6°C in 2 participants. This could be a manifestation of stress-induced hyperthermia. One participant had a forehead temperature 0.4°C lower than the baseline. In contrast, wrist temperature readings showed no obvious pattern, with temperature changes ranging from 0.1 to 1.1°C, and 72% (21/29) of the changes were larger than 0.2°C.
In summary, this study found that measuring distance (up to 10 cm) had a minimal impact on NCIT readings. Forehead readings were more consistent than wrist readings. Additionally, 9 minutes was sufficient for rewarming in 90% of participants after exposure to a cold outdoor environment. However, the study has several limitations. It was a single-institution study with a small sample size, enrolling only healthy young individuals. Only one NCIT model was evaluated, and the conclusions may not be applicable to other environmental conditions.
doi.org/10.1097/CM9.0000000000001546
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