Si-AQ-PRO Handheld Environmental Indoor Air Quality Monitor Customize to YOUR Needs. Measure 5 Parameters - % Relative Humidity, Temperature, Barometric and Differential Pressure, Wet Bulb & Dew Point. Choose UP TO 3 Gas Sensors Parameters - Carbon Dioxide (CO2), Carbon Monoxide (CO), Oxygen (O2),Formaldehyde (CH2O), Hydrogen Sulfur Dioxide (H2S), Nitric Oxide (NO), Nitrogen Dioxide (NO2) or Sulfur Dioxide (SO2).