The SCD-30 sensor reports the CO₂ levels in units of parts per million (ppm). This is a bit of a wacky unit of measure, but is pretty much what it says. If you had a million "parts" of air, then how many "parts" of CO₂ does it contain. Or as OSHA defines it:

Parts of vapor or gas per million parts of contaminated air by volume at 25 °C and 760 torr.

From the SCD-30 datasheet we can see that the sensor range is 400 to 10000 ppm:

But how do these ppm levels translate into air quality? Below is a PDF that covers lots of issues related to indoor air quality:

Buried in Appendix A is a section that discusses carbon dioxide levels. The key one they mention is the 5000 ppm Permissible Exposure Limit (PEL). Think of that as the upper limit - your workplace should ideally be below that level.

Values below the 5000 ppm PEL limit are commonly broken down into these subjective ranges, which we have also adopted for use in this project.

• < 1000 = Good air. Your body will be happy!
• 1000 - 2000 = Poor air. See if there is any way to improve.
• 2000 - 5000 =  Warning levels. Good idea to investigate why.
• > 5000 = Dang, you are above the OSHA Permissible Exposure Limit for 8 hour exposure. Something should be done.

So by "Dang" do you mean "Dangerous"? Sort of, but not like immediately dangerous. That requires levels greater than around 15,000 ppm (see PDF linked above). So it's not like you need to run for your life if the ppm jumps above 5000. But you don't want to spend extended periods of time in that environment.

Matrix Display of Air Quality

The general idea for the Matrix Portal based air quality display is pretty simple. It has a readout of the current CO₂ levels in ppm so that value it always viewable. Then, for each of the four ranges above, there is an associated "smiley face" icon and word. This combination of icon/word was chosen as way to make the display more universally readable. The smileys don't rely on any specific language and additionally use color to help reinforce the condition. The green=good and red=bad association is fairly universal. But not everyone can distinguish the red/green colors. So the word provides an additional textual (but in English only) indication in a single color.

Hopefully with the combination of the two, smiley + word, the message is universally understood.