How activated carbon removes gases from air
When air passes through an activated carbon filter, the gas and vapour molecules in the air are attracted to and held on the internal surface of the carbon by van der Waals forces. This is physical adsorption, not chemical reaction.
The carbon becomes progressively more saturated as it adsorbs these molecules. When the carbon surface is fully occupied, breakthrough occurs — the gas molecules pass through without being captured. At this point, the carbon filter must be replaced or regenerated.
What activated carbon removes from indoor air
- Volatile organic compounds (VOCs) — benzene, toluene, xylene, formaldehyde, acetaldehyde from building materials, furniture, and cleaning products
- Odours — cooking smells, pet odours, cigarette smoke, musty smells
- Chlorine gas — from tap water evaporation in showers
- Some industrial chemicals — solvents, fuel vapours
Formaldehyde is a special case — standard activated carbon adsorbs it relatively weakly. For high formaldehyde environments (newly built homes, furniture showrooms), impregnated carbon grades with specific reagents are more effective.
What activated carbon does NOT remove
- Particulates — dust, pollen, smoke particles, PM2.5 (HEPA filter required)
- Carbon dioxide — too small and too polar for physical adsorption on standard carbon
- Carbon monoxide — standard carbon is largely ineffective; catalytic treatment required
- Bacteria and viruses — carbon does not kill pathogens
Sizing and maintenance
The weight of activated carbon in a filter determines how long it lasts before breakthrough. Consumer air purifiers typically contain 100–300 g of carbon. Industrial air purifiers use kilograms of carbon in deep beds. Carbon filters should be replaced on the manufacturer’s schedule — a saturated filter provides no protection and may begin releasing previously adsorbed compounds.