The EPA determines the AQI scale and the HALO Smart Sensor provides an accurate measurement using multiple sensors. AQI readings are based on a number between 0-500 with certain intervals corresponding to different levels of health risks. 

• Green: A reading of 0-50 is considered good air quality and there is minimal to no risk. 
• Yellow: A reading of 51-100 is moderate air quality where there is a possibility of risk for those who have sensitivities to polluted air. 
• Orange: A reading of 101-150 is unhealthy for sensitive groups and they have a significant risk of developing adverse health effects. 
• Red: A reading of 151-200 is unhealthy for all groups of people and everyone is at risk of developing significant health effects. 
• Purple: A reading of 201-300 is very unhealthy for all groups and the risk for developing a health effect is very significant. 
• Maroon: A reading of 301 or higher is hazardous and anyone exposed to these conditions is greatly at risk of being affected. 

Reference: https://www.airnow.gov/aqi/aqi-basics/