https://techcrunch.com/2017/09/26/plume-labs-air-quality-tracker-is-now-available-for-pre-orders/

Plume Labs wants to be the definitive startup when it comes to air pollution. The company has built an app, an API and now an air quality tracker. The Flow is a tiny Bluetooth device you pair with your smartphone to learn more about the air you’re currently breathing.

It works both inside and outside and could be particularly useful if you live in a polluted city. It tracks particulate matter (PM2.5), nitrogen dioxide, ozone, volatile organic compounds (VOCs), temperature and humidity. You can clip it to your bag, bike or stroller. It has a tiny leather strap and looks nice.