If your device has already finished its calibration period, and is showing 0 / Zero after a few days of measuring, please go through the following steps:
Remove any dust / particles from the small air-holes at the bottom of the device. Ensure that the device is placed on a flat and smooth surface with the display facing upwards.
Replace the batteries, and wait a few days to see if the measurement changes.
If the device is still giving you a "0" reading then locate the serial number on the back of your device and get in touch with us at support@airthings.com.