Over the past few years there have been advances in sensing of some pollutants, primarily particles, that might lead to ventilation controls based on direct sensing of pollutants – particularly those relating to health. In this study we evaluated low-cost (about $200 US) IAQ monitors that measured PM2.5 - the most important health-related pollutant in indoor air. Controlled laboratory tests were carried out with known sources of particles (cooking, cleaning, candles, cigarettes) and by comparing the IAQ monitors response to research-grade and reference measurement methods. The results show a wide range of performance with the better IAQ monitors able to detect most particle sources/events and the worst ones detecting almost none. The best devices perform well enough to reliably operate a ventilation or filtration system but do not always report the correct magnitude of PM2.5 so there are restrictions on their use for calculating health effects.