First, United Nations officials label bacon and deli meats as carcinogens, and now scientists are claiming that higher concentrations of carbon dioxide are not only heating the planet, they’re making people dumber.

A new study by the Harvard School of Public Health claims “carbon dioxide (CO2) has a direct and negative impact on human cognition and decision-making,” according to ThinkProgress, a left-wing advocacy website.

Former Vice President Al Gore wasted no time tweeting out ThinkProgress’ in-depth article, trying to link higher carbon dioxide levels to lower cognition in humans.

Important piece by Joe Romm @thinkprogress on the direct, negative impact CO2 has on human cognition & decision making http://ow.ly/TS2jA

— Al Gore (@algore) October 26, 2015

The Harvard study exposed 24 people to office environments with varying levels of volatile organic compounds as well as varying levels of carbon dioxide over six work days. The study found that for “seven of the nine cognitive function domains, average cognitive scores decreased at each higher level of CO2.”

“Cognitive function scores were 15% lower for the moderate CO2 day (~945 ppm) and 50% lower on the day with CO2 concentrations around 1400 ppm than on the two Green+ days,” according to the Harvard study, which claims “green” buildings are better for your health.

ThinkProgress science writer Joe Romm took the study and is running a series of articles to promote his new book on global warming. Romm warns that “on average, a typical participant’s cognitive scores dropped 21 percent with a 400 ppm increase in CO2.”

“All of this new research is consistent with — and actually helps explain — literally dozens of studies in the past two decades that find low to moderate levels of CO2 have a negative impact on productivity, learning, and test scores,” Romm writes.

So is CO2 making us dumber? There are studies showing cognition decreases as CO2 levels increase, but there are also studies that show CO2 levels need to be extremely high to be harmful to human health. It’s also interesting that the U.S. Navy says average CO2 concentrations 3,500 parts per million (ppm) — levels nearly 10 times higher than what Harvard claims is safe.

“Data collected on nine nuclear-powered ballistic missile submarines indicate an average CO2 concentration of 3,500 ppm with a range of 0-10,600 ppm, and data collected on 10 nuclear-powered attack submarines indicate an average CO2 concentration of 4,100 ppm with a range of 300-11,300 ppm,” according to a 2007 National Research Council report on exposure issues facing submarine crews.

Interestingly enough, the NRC noted that a “number of studies suggest that CO2 exposures in the range of 15,000-40,000 ppm do not impair neurobehavioral performance.”

A study from 1961 exposed “23 crewmen exposed to CO2 at 15,000 ppm for 42 days in a submarine” and the men “showed no psychomotor testing effects but showed moderate increases in anxiety, apathy, uncooperativeness, desire to leave, and sexual desire.”

Another study from 1967 exposed seven men to CO2 concentration of 30,000 ppm. The men “reported no effects on hand steadiness, vigilance, auditory monitoring, memory, or arithmetic and problem solving performance.”

The NRC study goes on to highlight more studies finding to loss of cognitive abilities at CO2 levels many times higher than the Harvard study examined. NRC did link exposures to such high levels to headaches, tremors, slight increases in blood pressure and some other relatively minor effects.

A more recent 1998 study by Craig Idso, the founder of the Center for the Study of Carbon Dioxide and Global Change, found that people living in urban Phoenix, Arizona lived with CO2 levels as high as 555 ppm, and those lin more rural parts saw levels as high as 370 ppm.