New York, Researchers have found that rising atmospheric carbon dioxide (CO2) concentrations will cause urban and indoor levels of the gas to increase, which may significantly reduce our basic decision-making ability and complex strategic thinking.

Earlier research has shown that elevated CO2 levels can increase sleepiness and anxiety, and impair cognitive function.

The study, published in the journal GeoHealth, revealed that by the end of the century, people could be exposed to indoor CO2 levels up to 1400 parts per million - more than three times today's outdoor levels, and well beyond what humans have ever experienced.

"It affects everybody, from little kids packed into classrooms to scientists, business people and decision-makers to regular folks in their houses and apartments," study lead author Kris Karnauskas from the University of Colorado Boulder in the US.

According to the researchers, CO2 can also build up in poorly ventilated spaces over long periods of time, such as overnight while sleeping in bedrooms.

"When we breathe air with high CO2 levels, the CO2 levels in our blood rise, reducing the amount of oxygen that reaches our brains," said study researcher Shelly Miller.

Studies show that this can increase sleepiness and anxiety, and impair cognitive function.

Sitting too long in a stuffy, crowded lecture hall or conference room and many of us begin to feel drowsy or dull. In general, CO2 concentrations are higher indoors than outdoors, the authors wrote. And outdoor CO2 in urban areas is higher than in pristine locations.

The CO2 concentrations in buildings are a result of both the gas that is otherwise in equilibrium with the outdoors, but also the CO2 generated by building occupants as they exhale.

In the ongoing scenario in which people on Earth do not reduce greenhouse gas emissions, the Intergovernmental Panel on Climate Change predicts outdoor CO2 levels could climb to 930 ppm by 2100. And urban areas typically have around 100 ppm CO2 higher than this background.

For the findings, the research team developed a comprehensive approach that considers predicted future outdoor CO2 concentrations and the impact of localized urban emissions, a model of the relationship between indoor and outdoor CO2 levels and the impact on human cognition.

They found that if the outdoor CO2 concentrations do rise to 930 ppm, that would nudge the indoor concentrations to a harmful level of 1400 ppm.

"At this level, some studies have demonstrated compelling evidence for significant cognitive impairment," said study researcher Anna Schapiro.

"Though the literature contains some conflicting findings and much more research is needed, it appears that high level cognitive domains like decision-making and planning are especially susceptible to increasing CO2 concentrations," Schapiro added.

In fact, at 1400 ppm, CO2 concentrations may cut our basic decision-making ability by 25 per cent, and complex strategic thinking by around 50 per cent, said study researchers.