In 2012, air pollution in both cities and rural areas was estimated to have caused 3.7 million premature deaths, 88% of those in at-risk communities. The primary pollutant was small airborne particulate matter of 10 microns or less in diameter, which led to the development of cardiovascular and respiratory diseases. In response, we developed MyPart, the first personal, portable, and accurate particle sensor under $50 capable of distinguishing and counting differently sized particles. We demonstrate how MyPart offers substantial enhancements over most existing air particle sensors by simultaneously improving accessibility, flexibility, portability, and accuracy. We describe the evolution and implementation of the sensor design, demonstrate its performance across twenty everyday urban environments versus a calibrated instrument, and conduct a preliminary user study to report on the overall user experience of MyPart. We also present a novel smart-phone visualization interface and a series of simple form factor adaptations of our design.