A study published by the journal Environmental Health Perspectives in 2012 found that CO2 levels of 2,500 parts per million (ppm) cause "large and statistically significant reductions" in high-level brain functions like initiative, strategic thinking, and complex decision-making. A study published by the journal Nature in 2018 found similar results. Average CO2 concentrations in the breathing zones of masks approach or exceed that level. For example, a study published in 2021 by the journal Aerosol and Air Quality Research found that the average CO2 concentration was 2,051 ppm in a 3-layer cloth mask and 2,107 ppm in a surgical mask. This study was done in a ventilated lab with CO2 levels close to that of fresh air, but in the real world, indoor CO2 levels are often much higher, which may push the CO2 concentrations in the masks to well beyond what the study found. The CO2 levels in N95 masks commonly worn in hospitals are far higher, ranging from 13,000 to 35,000 ppm.