Quick answer
Two filters do the work. A HEPA filter traps the particles in cannabis smoke. An activated carbon filter adsorbs odor and gases. A HEPA filter alone removes the haze and leaves the smell. A carbon filter alone leaves the particles. Use both.
What cannabis smoke contains
Smoke from a joint, pipe, or bong carries particles and gases. Particle sizes run from about 0.1 micron to 1 micron. The gas phase holds terpenes, volatile organic compounds, formaldehyde, and acrolein. A HEPA filter is rated to capture 99.97 percent of particles at 0.3 micron. It captures larger and smaller particles at similar or higher rates.
Filter types compared
- HEPA, H13 or true HEPA: particles. 99.97 percent at 0.3 micron. No odor removal.
- Activated carbon: gases and odor. Carbon mass matters. A 1 lb carbon bed lasts longer than a 3 oz pad.
- MERV 13 to MERV 16: furnace and HVAC filters. MERV 13 catches smoke particles. MERV 16 sits close to HEPA.
- Electrostatic precipitators: collect particles on charged plates. They make small amounts of ozone.
- Ozone generators: skip them. EPA advises against ozone generators in occupied spaces.
Sizing a filter for a room
Use cubic feet and air changes per hour (ACH). Formula: CFM = (room cubic feet x ACH) / 60.
Example: a 12 ft by 12 ft room with an 8 ft ceiling holds 1,152 cubic feet. At 6 ACH, the filter needs 115 CFM.
HEPA Filter for Marijuana Smoke: A Comprehensive Guide
- Light use: 4 to 6 ACH.
- Daily smoking: 6 to 10 ACH.
- Commercial space or multiple smokers: 10 ACH and up.
CADR on the box is measured at the top fan speed. A unit rated 120 CFM moves less air on low. Noise rises with fan speed.
Maintenance schedule
Pre-filter: rinse or replace every 1 to 2 months. HEPA: replace every 12 to 18 months, sooner in a smoky room. Carbon: replace every 3 to 6 months. Carbon saturates and stops adsorbing. It does not change color in every case, so track the install date.
What filters do not fix
Combustion makes carbon monoxide. A HEPA filter and a carbon filter do not remove it in useful amounts. Carbon monoxide needs exhaust or a detector. Smoke also leaves residue on walls, glass, and fabric. Filters slow that buildup. They do not stop it.
Exhaust beats filtration for odor control. A fan that pushes air outdoors removes smoke at the source. Run an exhaust fan plus a filter for the best result.