Air quality sensors are specialized instruments designed to detect and quantify substances in the air that can impact human health and the environment. Ranging from simple consumer monitors to sophisticated industrial units, these devices are becoming a common tool for understanding the immediate environment as awareness of air quality grows. Sensors focus on measuring two broad categories of pollution: solid and liquid particles suspended in the air, and various gaseous chemical compounds.
Particulate Matter Measurements
Particulate matter (\(\text{PM}\)) represents a complex mixture of solid particles and liquid droplets suspended in the air. These particles are classified and measured based solely on their physical size, as this dimension determines how deeply they can penetrate the human respiratory system. Air quality sensors typically focus on two distinct size fractions, \(\text{PM}_{10}\) and \(\text{PM}_{2.5}\).
\(\text{PM}_{10}\) refers to inhalable particles with diameters generally 10 micrometers or less, including dust, pollen, and mold spores. These coarse particles are typically trapped in the upper respiratory tract, such as the nose and throat, but can still affect the heart and lungs. \(\text{PM}_{2.5}\) is more concerning because it includes fine particles that are 2.5 micrometers or less in diameter, which is about one-thirtieth the width of a human hair.
The smaller size of \(\text{PM}_{2.5}\) allows it to travel much deeper into the lungs, where it can enter the bloodstream. Sources of these fine particles often include combustion processes, such as vehicle exhaust, wildfires, and wood burning. Long-term exposure to \(\text{PM}_{2.5}\) has been linked to severe respiratory and cardiovascular problems.
Detecting Key Gaseous Pollutants
Beyond particles, air quality sensors are engineered to detect a range of gaseous pollutants, each with different sources and health implications. Carbon monoxide (\(\text{CO}\)) is a colorless, odorless gas produced by the incomplete combustion of carbon-containing fuels. Sources include vehicle exhaust, gas stoves, and faulty furnaces, and high concentrations can be lethal because \(\text{CO}\) interferes with the blood’s ability to carry oxygen.
Nitrogen dioxide (\(\text{NO}_2\)) is primarily generated from the burning of fuel in motor vehicles and industrial activities. While \(\text{NO}_2\) itself can irritate the respiratory system, it is also a precursor to the formation of other harmful pollutants, including ground-level ozone and \(\text{PM}\).
Ozone (\(\text{O}_3\)), specifically ground-level ozone, is not directly emitted but is formed when nitrogen oxides and volatile organic compounds react in the presence of sunlight. This gas is a powerful lung irritant and is a major component of smog.
Volatile Organic Compounds (\(\text{VOCs}\)) are a diverse group of carbon-containing chemicals that easily evaporate at room temperature. Sensors often report total \(\text{VOCs}\) (\(\text{TVOCs}\)) which can come from common indoor sources like paints, cleaning supplies, air fresheners, and furnishings. Although individual \(\text{VOCs}\) vary in toxicity, high concentrations can cause short-term effects like headaches and nausea, and some are linked to long-term health issues.
Sensor Technology and Data Reporting
The measurements of particles and gases rely on distinct physical and chemical principles employed by the sensors. Particulate matter measurements, for example, are typically performed using laser scattering technology. This method involves drawing air into a chamber where a laser beam is directed through the sample; airborne particles scatter the light, and a detector measures the intensity and pattern of this scattered light to determine the particle count and size.
Gaseous pollutants are often measured using electrochemical sensors or metal oxide semiconductor (\(\text{MOS}\)) sensors. Electrochemical sensors work by allowing a specific gas to react with a chemical solution, generating an electrical signal proportional to the gas concentration. \(\text{MOS}\) sensors use a heated metal oxide film whose electrical conductivity changes when it interacts with gases like \(\text{VOCs}\), providing a measurable signal.
Raw concentration data from all these sensors is most often translated into a single, easy-to-understand metric called the Air Quality Index (\(\text{AQI}\)). The \(\text{AQI}\) converts complex pollutant concentrations for substances like \(\text{PM}_{2.5}\), \(\text{O}_3\), and \(\text{CO}\) into a single numerical scale, typically ranging from 0 to 500. The overall \(\text{AQI}\) is determined by the pollutant that poses the greatest risk at that moment.
This index is categorized into six color-coded levels that communicate the associated health risks. For instance, a Green \(\text{AQI}\) (0-50) signifies “Good” air quality, posing little risk, while an Orange \(\text{AQI}\) (101-150) indicates “Unhealthy for Sensitive Groups”. Readings that reach Purple (201-300) or Maroon (301+) levels are considered “Very Unhealthy” or “Hazardous,” respectively, indicating increased health risks for the entire population.