http://explorables.cmucreatelab.org/explorables/air-quality-monitor-tests/

https://smartairfilters.com/en/blog/accurate-1-micron-dylos-official-pm2-5-comparison-test/

https://cleantechnica.com/2016/07/09/home-indoor-air-quality-monitors-best/

https://www.amphenol-sensors.com/en/telaire/dust-sensors/3393-sm-uart-04l

https://towardsdatascience.com/sensing-the-air-quality-5ed5320f7a56

According to an article in eldiario.es (https://www.eldiario.es/madrid/viajeros-Metro-Madrid-respiran-contaminacion_0_876662709.html), airBeam1 PM2.5 offers very good results.

Wearable sensor: https://www.rti.org/impact/micropem-sensor-measuring-exposure-air-pollution

https://www.instructables.com/How-to-Build-a-Portable-Accurate-Low-Cost-Open-Sou/

https://www.researchgate.net/publication/325330102_Improving_a_Commodity_Dust_Sensor_to_Enable_Particle_Size_Analysis

https://www.breeze-technologies.de/air-quality-sensors/

Kaiterra

HoldPeak HP5800-D It seems to be good, but not able to do continuous measurement.

Temtop M10 Cheap. Recommended by nytimes review. Perhaps also M1000.

PM-1064SD

https://luftbewusst.de/luftqualitaet-messgeraet/

https://www.techhive.com/article/3356448/best-indoor-air-quality-monitor.html

https://www.nytimes.com/wirecutter/reviews/best-home-air-quality-monitor/