Recently, a number of optical particulate matter (PM) monitors employing low-cost PM sensors have become available on the consumer market. These portable low-cost monitors can be used to characterize PM concentrations with high spatial and temporal resolution. This study evaluates the performance of four low-cost PM monitors (Speck, Dylos, TSI AirAssure, and UB AirSense) against well-characterized reference instruments, and studies their suitability for PM field exposure studies. The low-cost monitors were characterized in a room-sized laboratory chamber with standard relative humidity and temperature conditions, with two PM sources: cigarette smoke and Arizona Test Dust. This study found that any of the monitors tested perform with adequate precision for monitoring air quality in an indoor microenvironment, although the field calibration of the monitor with a standard instrument for specific types of particles would be required. Other factors such as flexibility in data download methods; connectivity, compatibility with environmental conditions, and quality of technical support should also be considered when selecting low-cost PM monitors for human inhalation exposure assessment studies. (C) 2016 Elsevier Ltd. All rights reserved.