Xiaomi Portable Air Quality sensor

The portable air quality sensor definitely isn't available from the UK Mi store, but importable.

The unit measures about 6 x 9 x 2 cm and is a plain white case with an OLED display that shows the battery charge and the PM2.5 value. It's small enough to fit in a jean's or jacket pocket.

The sensor is branded SmartMi.

There's a button on the front that turns the unit on or off and a micro-USB socket for charging and that's it. Short pressing the button while the display is on will cycle between brightness settings (low, medium, high).

While it was sensible to do so, using it on the tube (is that ever sensible?) produced interesting results, especially on the Northern Line where PM2.5 levels could reach over 160ppm (not safe) in the deeper stations. The display will show a bar that's green, orange or red depending on the PM2.5 level 0-75, 76-150 and 150+ respectively.

Internally there's fan and a laser reflection/scatter system that measures the particulates.

Available through your Chinese import site for about £20.