An air purifier is an engineered system that improves indoor air quality by moving room air through materials designed to remove contaminants. It can reduce airborne dust, pollen, smoke particles, pet dander, and some odor-causing gases. Its effectiveness depends on both the filters and the rate at which the device circulates air.
A purifier does not create oxygen or remove every pollutant, but it can lower the concentration of many indoor contaminants.
A fan creates a pressure difference that draws dirty room air into the intake. A pre-filter catches large lint, hair, and dust, protecting the finer filters behind it. A HEPA filter traps very small particles in a dense fiber network, while activated carbon adsorbs many odor molecules and volatile gases onto its porous surface.
The fan then pushes the filtered air out through the exhaust, where it mixes back into the room and gradually reduces pollutant levels.
Key Facts
- Airflow pathway: room air in → pre-filter → HEPA filter → activated carbon → fan → clean air out.
- The fan creates a pressure difference, ΔP, that drives air through the purifier.
- Volumetric airflow rate is Q = V/t, where V is air volume moved in time t.
- CADR = airflow rate × single-pass particle removal efficiency.
- A HEPA filter is commonly rated to capture at least 99.97% of 0.3 μm test particles under its specified test conditions.
- Activated carbon removes many gases by adsorption, in which molecules stick to the large internal surface area of porous carbon.
Vocabulary
- Pre-filter
- A first-stage filter that captures large particles such as hair, lint, and visible dust before they reach finer filters.
- HEPA filter
- A high-efficiency fibrous filter designed to capture very small airborne particles through several physical trapping mechanisms.
- Activated carbon
- A highly porous form of carbon that adsorbs many odor molecules and gaseous pollutants onto its surface.
- CADR
- Clean Air Delivery Rate, a measure of how quickly an air purifier delivers cleaned air for a particular type of particle.
- Pressure difference
- The difference in air pressure between two locations that causes air to flow from higher pressure toward lower pressure.
Common Mistakes to Avoid
- Assuming a HEPA filter removes odors and gases, which is wrong because HEPA media is mainly designed for particles; activated carbon is the stage that adsorbs many gases and odors.
- Placing the purifier tightly against a wall or behind furniture, which is wrong because blocked intakes or exhausts reduce airflow and lower cleaning performance.
- Treating CADR as the same as fan airflow, which is wrong because CADR accounts for both airflow and how effectively the purifier removes particles in one pass.
- Never replacing a loaded filter, which is wrong because accumulated particles and saturated carbon increase resistance, reduce airflow, and can reduce pollutant removal.
Practice Questions
- 1 A purifier moves 180 m^3 of air in 30 minutes. Calculate its volumetric airflow rate in m^3/h.
- 2 A purifier has an airflow rate of 250 m^3/h and removes 80% of a certain particle type in one pass. Calculate its CADR for that particle type.
- 3 A purifier has a clean HEPA filter but no activated carbon layer. Explain why it may remove smoke particles well while a smoke odor can still remain in the room.