This channel (https://thingspeak.com/channels/322661) reports the CO2 or pollutant level returned by an MQ-135 sensor. You can compare this with the average atmospheric CO2 level available at https://www.co2.earth.

Co2 Monitoring and Simulation Linking

Измерение CO2

Monitor Room T&H,CO2,TVOC

verifies industrial air pollution

Indoor/Outdoor Air Quality Monitoring Particle, VOCs, CO2, Temperature

Temprature data

Efftronics IOT

Hi, you can see CO2 concentration in our office meeting room (data sampled per 15 seconds)

Monitor greenhouse gas

Medición de temperatura, humedad y concentración de CO2

Ligar arduino à internet