Skip to main content
Assessing Indoor Versus Outdoor PM2.5 Concentrations During the 2025 Los Angeles Fires Using the PurpleAir Sensor Network

Assessing Indoor Versus Outdoor PM2.5 Concentrations During the 2025 Los Angeles Fires Using the PurpleAir Sensor Network

This is a Preprint and has not been peer reviewed. This is version 1 of this Preprint.

Add a Comment

You must log in to post a comment.


Comments

There are no comments or no comments have been made public for this article.

Downloads

Download Preprint

Authors

Yan Lu, Xinyi Zhang, Soroush Esmaeili Neyestani, Xiang Li, Ling Jin, Lu Zhang, Rima Habre, Jiachen Zhang 

Abstract

In January 2025, a series of fast-moving wildland-urban-interface (WUI) fires swept through the Los Angeles (LA) metropolitan area, causing severe air pollution. While the impacts of WUI fires on outdoor air quality have been extensively studied, indoor exposure remains less understood, despite most people sheltering indoors during WUI fires. This study investigates the spatial and temporal patterns of indoor and outdoor PM₂.₅ concentrations across the South Coast Air Basin, with a focus on Los Angeles County during the LA fires. Using high-resolution data from co-located indoor and outdoor PurpleAir sensors, we analyze hourly PM₂.₅ levels and indoor/outdoor ratios. Outdoor PM₂.₅ concentrations spiked sharply during the fires, reaching unhealthy levels. Indoor concentrations increased concurrently but to a lesser extent, reflecting the partial shielding effect of indoor environments from outdoor air pollution. The mean daily indoor/outdoor PM₂.₅ ratio was 0.50 during LA fire days, lower than that ratio (0.81) during non-fire days. Indoor/outdoor PM₂.₅ ratios across sensors showed a wide distribution, reflecting differences in building characteristics and occupant behavior, such as the use of air purifiers. These findings emphasize the need for guidance and interventions to reduce indoor PM ₂.₅ exposure and protect public health during extreme WUI fire events.

DOI

https://doi.org/10.31223/X59M9T

Subjects

Engineering

Keywords

PurpleAir low-cost sensors, indoor air quality, wildland-urban-interface (WUI) fire, PM2.5, 2025 Los Angeles Fires, indoor air quality, wildland-urban-interface (WUI) fire, PM2.5, 2025 Los Angeles Fires

Dates

Published: 2025-07-30 09:16

Last Updated: 2025-07-30 09:16

License

CC BY Attribution 4.0 International

Additional Metadata

Conflict of interest statement:
None

Data Availability (Reason not available):
PurpleAir data used in this analysis is publicly available upon purchase.