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.
Cannabis Smoke Scrubber: The Ultimate Guide
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.
cannabis air filter for grow room
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.
The Ultimate Buying Guide to HEPA Filters for Marijuana Smoke
- 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.