Calibration of indoor temperature and relative humidity readings in the PurpleAir monitor

Kyle T Aune; Megan Wilks; Timothy Green; Ana M Rule; Meredith McCormack; Nadia N Hansel; Nirupama Putcha; Gregory Kirk; Sarath Raju; Kirsten Koehler

doi:10.1007/s10661-025-14076-5

Calibration of indoor temperature and relative humidity readings in the PurpleAir monitor

Environ Monit Assess. 2025 May 22;197(6):667. doi: 10.1007/s10661-025-14076-5.

Authors

Affiliations

¹ Department of Environmental Health and Engineering, Johns Hopkins Bloomberg School of Public Health, Baltimore, MD, USA.
² Division of Pulmonary & Critical Care Medicine, Johns Hopkins University, Baltimore, MD, USA.
³ Department of Epidemiology, Johns Hopkins Bloomberg School of Public Health, Baltimore, MD, USA.
⁴ Department of Epidemiology and Medicine, Johns Hopkins University, Baltimore, MD, USA.
⁵ Department of Environmental Health and Engineering, Johns Hopkins Bloomberg School of Public Health, Baltimore, MD, USA. kkoehle1@jhu.edu.

PMID: 40402320
DOI: 10.1007/s10661-025-14076-5

Abstract

Temperature extremes are associated with a variety of negative health outcomes, but since people living in developed areas of the world spend most of their time indoors, outdoor temperatures are a poor substitute for personal exposure assessment. And the importance of accurate indoor temperature measurement has only become more apparent alongside the growing impact of climate change on the frequency and intensity of temperature extremes on public health. The development and implementation of low-cost sensors have improved economic and practical feasibility of in-home exposure assessment for temperature and a variety of indoor contaminants. One example is the PurpleAir particulate matter sensor. However, onboard electronics likely bias measured temperature (T) and relative humidity (RH) values in such devices. The objectives of this investigation were to (1) characterize bias and error of T and RH estimates by comparing 30,936 hourly mean values to a dedicated, calibrated temperature and RH sensor co-located at 115 homes in Baltimore, MD, (2) develop calibration equations to accurately approximate the reference values, (3) validate the performance of these equations, and (4) validate the transportability of these calibration equations using 7697 pairs of hourly mean temperature and RH values from 22 homes in western Maryland. The PurpleAir sensors measured higher temperatures and lower RH than indoor reference measurements. Calibration schemes using a bias correction and multiple linear regression were considered. Calibration of PurpleAir temperature in °C (T_PA) may be performed by T_indoor = T_PA - 3.95 °C (R² = 0.80), and calibration of PurpleAir RH measured from 0-100% (RH_PA) may be performed by RH_indoor = - 0.29 + 1.39∙RH_PA (R² = 0.92). Calibration-corrected temperature and RH demonstrated low bias and error values, and these results were consistent in the withheld datasets.

Keywords: Calibration; PurpleAir; Sensor; Temperature.

MeSH terms

Air Pollutants / analysis
Air Pollution, Indoor* / analysis
Air Pollution, Indoor* / statistics & numerical data
Calibration
Environmental Monitoring* / instrumentation
Environmental Monitoring* / methods
Humidity*
Particulate Matter / analysis
Temperature*

Substances

Particulate Matter
Air Pollutants

Abstract

MeSH terms

Substances

Grants and funding