You can buy it at online shop like eBay or aliexpress about 20$.
The electrical interface of this sensor is compatible with Raspberry Pi. So you can make your D.I.Y. CO2 Monitor with MH-Z19, Raspberry Pi .
Normal CO2 concentration is about 400ppm. On a global scale, this value is sensitive to the effect of the activities of the plants, seasonally it get lower in summer due to vigorous plants photosynthesis and getting higher in winter, and the long range trend is continue to rise linearly. The Japanese Meteorological Agency report CO2 concentration chart continuously as hire. It indicate clear trend of rising.
On your room scale, in a closed room with 2 person, CO2 concentration rise up to 1,200ppm soon, and keep falling slowly after 1 person go out.
In high CO2 concentration room as 1200ppm, somebody can get a headache or sleepiness.
But no need to concern about it. It fall dow…