Indoor levels of carbon dioxide could be clouding our thinking and may even pose a wider danger to human health, researchers say.

While air pollutants such as tiny particles and nitrogen oxides have been the subject of much research, there have been far fewer studies looking into the health impact of CO 2 .

However, the authors of the latest study – which reviews current evidence on the issue – say there is a growing body of research suggesting levels of CO 2 that can be found in bedrooms, classrooms and offices might have harmful effects on the body, including affecting cognitive performance.

“There is enough evidence to be concerned, not enough to be alarmed. But there is no time to waste,” said Dr Michael Hernke, a co-author of the study from the University of Wisconsin-Madison, stressing further research was needed.

Writing in the journal Nature Sustainability, Hernke and colleagues report that they considered 18 studies of the levels of CO 2 humans are exposed to, as well as its health impacts on both humans and animals.

Traditionally, the team say, it had been thought that CO 2 levels would need to reach a very high concentration of at least 5,000 parts per million (ppm) before they would affect human health. But a growing body of research suggests CO 2 levels as low as 1,000ppm could cause health problems, even if exposure only lasts for a few hours.

The team say crowded or poorly ventilated classrooms, office environments and bedrooms have all been found to have levels of CO 2 that exceed 1,000ppm, and are spaces that people often remain in for many hours at a time. Air-conditioned trains and planes have also been found to exceed 1,000ppm.

“Indoor environments are of much more concern presently and for many people that is where they spend 60-80% of their time,” said Hernke, although projections suggest by 2100 some large cities might reach outdoor CO 2 levels of 1,000ppm for parts of the year.

The team found a number of studies have looked at the impact of such levels on human cognitive performance and productivity. In one study of 24 employees, cognitive scores were 50% lower when the participants were exposed to 1,400ppm of CO 2 compared with 550ppm during a working day.

The team additionally looked at the impact of CO 2 levels on animals, finding that a few hours’ exposure to 2,000 ppm was linked to inflammatory responses that could lead to damage to blood vessels. There is also tentative evidence suggesting that prolonged exposure to levels between 2,000 and 3,000ppm is linked to effects including stress, kidney calcification and bone demineralisation.

The team add that rising outdoor levels of CO 2 will mean rising indoor levels – a situation that could be exacerbated by greater use of certain air-conditioning units, people spending more time inside, energy-saving building techniques, and increasing urbanisation.

Any health impacts, they add, might be particularly problematic for children or those with health conditions that might exacerbate the effects. And even if the impacts are reversible, said Hernke, it would depend on people being able to access air with low levels of CO 2 . “The question is what happens over the very long term when you are unable to go outside and, as it were, have that carbon sucked back out of you?”