This document summarizes a review of low-cost air quality sensors. It analyzed sensors for PM, O3, NO2, NO and CO based on their agreement with reference measurements, ability to measure multiple pollutants, data availability, and value. Sensors were shortlisted that showed good agreement (R2 > 0.85, 0.8 < slope < 1.2) including models from AirNut, PA-I, PA-II, Egg and others ranging from $132-880. While few sensors were open source, most were "black boxes" only measuring one pollutant. The review concluded there is a need to target the best sensor based on openness, agreement, number of pollutants and