If someone smokes cannabis inside, the whole house breathes it. In a two-story California house, one joint smoked in the living room raised fine particles in every room, and the two upstairs bedrooms with open doors ran about as high as the living room itself.
The one room that stayed low had its door shut. Opening three windows cut the smoke by two thirds, and the house still carried more than a tobacco cigarette leaves behind with the windows closed.
The study
Wayne Ott, Lance Wallace, Kai-Chung Cheng and Lynn Hildemann, three of them from Stanford’s civil and environmental engineering department, published the study in Science of the Total Environment in December 2022. It was funded by the University of California’s Tobacco-Related Disease Research Program, and the authors declare no competing interests.
They set a calibrated real-time monitor for PM2.5, the fine particles that reach deep into the lungs, in the middle of each of the 9 major rooms of a 4-bedroom, 2,155-square-foot house in Redwood City, California, built in 1986. A smoking machine in the living room fully smoked either a Marlboro cigarette or a commercial pre-rolled joint, with the same puff settings for both. Nobody was inside. The heating and air conditioning were off. Seven 5-hour runs took place between May 2020 and July 2021.
What they found
With the windows closed, the two cigarettes produced 5-hour averages of 15.2 and 15.0 micrograms per cubic meter across the 9 rooms. The three joints produced 38.9, 79.8 and 80.7.
On average the joints came out 4.4 times the cigarettes, and a joint beat a cigarette in every room of the house.
Those joint numbers are already scaled down, because the joints weighed more than the cigarettes. The authors say the raw readings run up to 54% higher than the published ones. The result also lines up with the team’s earlier emission-rate study, where 24 joints averaged 3.5 times the emissions of 9 Marlboro cigarettes.
For one joint, the living room averaged 105.3, which is 5.8 times the cigarette in the same room. The two upstairs bedrooms averaged 98.8 and 98.1. The downstairs bedroom, reached through a short, low hallway, averaged 64.3.
The master bedroom was kept shut, as the family who lives there keeps it. Across the three joints it averaged 12.8 to 24.1. In those same runs, every other room ranged from 36.4 to 106.8.
Why it spreads
Once the smoke mixed, every room except the shut one behaved like a single space, which the authors call a single mixing compartment. Warm air rises, and the stairway starts right beside the spot where the joint burned, which helps explain the high upstairs readings.
The shut door still leaked. It had a 1.9 cm gap, about three quarters of an inch, at the floor. An earlier tracer-gas study in this same house found the downstairs bedroom door, closed, let through 4% to 7% of the tracer gas, and a towel stuffed underneath stopped essentially all of it.
The limits, stated plainly
- One house. A machine did the smoking, and nobody was breathing the air.
- A small sample. Two cigarettes and five joints.
- The furnace was off. A running furnace fan would likely move smoke between rooms differently. That is our inference, and this study did not test it.
- Particles only. No THC, no other chemicals, and no one’s health was measured.
- Joints only. Vapes and edibles were not part of it.
- One run each for the window tests, each with a different joint.
What survives: one joint in one room raised fine particles throughout an ordinary house, far more than a cigarette did.
What changes it
Windows helped. With one window open 12.7 cm, about 5 inches, the whole-house average fell by 30%. With three windows open the same amount, it fell by 67%, to 21.9, and that was still 45% higher than a cigarette with every window shut.
A closed door protected the room behind it. In the earlier tracer test in this house, a towel at the gap did even better.
What gets lost in the smoke-odor marketing
Odor sprays, candles and ozone “smoke eliminator” machines are all sold for this problem. The EPA’s page on ozone generators sold as air cleaners, last updated February 19, 2026, says ozone “does not remove particles” from the air, and that at levels within public health standards ozone is “generally ineffective” at controlling indoor pollution. Covering the smell leaves the particles in the air.
The same EPA page names the AHAM clean air delivery rate, which includes a tobacco smoke rating, as a way to judge portable air cleaners. This study did not test an air cleaner.
What the data supports
- Before anything was lit, the house read under 4. Not smoking indoors keeps it there. Nothing tested here brought a joint’s whole-house average down to a cigarette’s.
- If it happens anyway, keep the doors shut to rooms where people sleep, and put a towel at the gap.
- Open windows in more than one room. In single runs, three windows cut the average by 67% and one window by 30%.
- Mind the stairs. If they are near where the smoking happens, expect the upstairs to get it.
Bottom line
One joint in the living room reaches every room with an open door, for hours. A shut door kept most of it out of one bedroom.
First step: tonight, close the bedroom doors of anyone in the house who does not smoke.
illumenair measures PM2.5 room by room in Portland and Vancouver homes. Call 971-363-5626, email info@illumenair.com or visit www.illumenair.com.
Reference: Ott WR, Wallace LA, Cheng K-C, Hildemann LM. “Measuring PM2.5 concentrations from secondhand tobacco vs. marijuana smoke in 9 rooms of a detached 2-story house.” Science of the Total Environment, 2022;852:158244. doi:10.1016/j.scitotenv.2022.158244.