There are several reasons why formaldehyde readings on your air quality monitor did not change, even after using a carbon filter. The most likely reason is that your formaldehyde monitor actually cannot detect formaldehyde.
Although often marketed and sold as formaldehyde testers or detectors, these devices cannot measure a single type of gas (formaldehyde). To test whether your formaldehyde tester is also testing for other gases, try blowing gently into the formaldehyde tester. Most likely, you fill find that the tester’s formaldehyde reading will increase. This shows the tester is not measuring just formaldehyde, but a range of gases called ‘VOCs’.
A second reason for formaldehyde reading issues is that temperature and humidity have a large influence on the measured value VOC or formaldehyde testers give. In general, we do not recommend using formaldehyde testers. However, if you are using one, then we recommend measuring when temperature and humidity are constant.