Higher Indoor CO2 Levels: Good for Growing Cannabis, Bad for Human Cognition

Joseph J. Romm, Ph.D.By Joseph J. Romm, Ph.D.

Cannabis growth is generally enhanced by higher levels of indoor carbon dioxide. That’s why indoor grow areas typically increase CO2 levels by a factor of two to four from outdoor levels.

For decades, it was widely believed that such moderately higher CO2 levels had no negative impact on humans. But in a landmark public health finding, a 2015 Harvard School of Public Health study found that CO2 at such levels in fact have a direct and negative impact on human cognition and decision-making.

Outdoor CO2 levels are currently around 400 parts per million – a rise from preindustrial levels of 280 ppm driven by burning fossil fuels. Since plants like cannabis grown under controlled conditions can have higher yields with elevated CO2 levels, growers typically boost indoor levels to between “700 and 1500 PPM,” according to the Colorado Department of Public Health and Environment (CDPHE).

In a 2015 presentation, the CDPHE repeated the long-held view that this is not problem because the upper acceptable Occupational Exposure Limit (OEL) for CO2 is 5,000 ppm. Other experts have similarly written that CO2 levels of 700 to 1500 ppm are “are not of public health concern.”

Unfortunately, recent research has overturned this conventional wisdom. Consider a 2012 Lawrence Berkeley National Laboratory (LBNL) study, “Is CO2 an Indoor Pollutant? Direct Effects of Low-to-Moderate CO2 Concentrations on Human Decision-Making Performance.” It found “statistically significant and meaningful reductions in decision-making performance” in test subjects as CO2 levels rose from a baseline of 600 parts per million (ppm) to 1000 ppm and higher.

Both the Harvard and LBNL studies made use of a sophisticated multi-variable assessment of human cognition. The Harvard School of Public Health researchers used a lower CO2 baseline than LBNL. They found that, on average, a typical participant’s cognitive scores dropped 21 percent with a 400 ppm increase in CO2.

Here are their astonishing findings for four of the nine cognitive functions scored in a double-blind test of the impact of elevated CO2 levels:

CO2 Concentration (ppm)

They conclude, “The largest effects were seen for Crisis Response, Information Usage, and Strategy, all of which are indicators of higher level cognitive function and decision-making.”

NASA has also observed CO2-related health impacts on space-station astronauts at much lower CO2 levels than expected and has identified a mechanism by which CO2 levels could affect the brain, as I reported last year. But we still don’t know the answer to key questions like “What are the long-term health impacts of exposures to moderate CO2 levels” and “Are certain groups or individuals more vulnerable to moderate CO2 levels?”

The bottom line is that if you have a grow room with elevated levels of CO2, the best new research says you should minimize the time you spend in it.

Finally, studies show that many other spaces routinely have CO2 elevated to levels that can negatively impact productivity, performance and test scores, including office buildings, homes, air plane cabins, cars and school classrooms. I measured CO2 levels as high as 2500 ppm at a recent all-day conference! So you may want to consider purchasing a portable CO2 monitor for everyday use.

Joseph J. Romm, Ph.D.

Dr. Joseph Romm is a former acting Assistant Secretary for energy efficiency and renewable energy. He is Chief Science Advisor for the Emmy-winning docu-series “ Years of Living Dangerously,” whose Season 2 airs on National Geographic Channel starting October 30. He is founding editor of the website ClimateProgress.org. Dr. Romm is an advisor to New Frontier Financial. The views expressed here are his own.

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