Indoor air pollution and cognitive impairment

The ill effects of air pollution on one’s health are well known amongst the general public, and higher rates of respiratory issues in regions with significant air pollution, such as Delhi, are no secret. Research in the past few years has suggested that these negative effects may not be limited to the respiratory system. Accumulating evidence suggests that exposure to pollutants could impact our brain and, in turn, our cognition. It has also been cited as a contributing factor to dementia.

While we are generally well aware of what constitutes and contributes to outdoor air pollution, indoor air pollutants are rarely discussed. As it turns out, a number of things that we would typically consider benign, such as burning incense sticks, could actually be harming us!

To put it in context, one paper described incense sticks as releasing almost five times as much particulate matter as cigarettes. Studies focusing specifically on cognition also report a small but significant negative effect of incense smoke exposure on cognitive screening measures. While the small effect sizes may not be sufficient to raise concern, it is important that we also consider the impact of other sources of indoor air pollution on cognition.

One of the most common sources of indoor air pollution is exposure to fumes from solid fuel use (for cooking or heating purposes). A Lancet report from 2022 suggests that more than 60% of individuals in low- and middle-income countries are exposed to unclean fuel. Past reports have associated this exposure with a host of poor physical health outcomes, including but not limited to low birth weight and infant mortality. A study from Bangladesh also found this source of household air pollution to be a risk factor for developmental delay. Exposure to fumes from solid fractions of biogas also resulted in lower scores on a test of arithmetic, relative to children residing in households that used clean fuels such as liquid petroleum gas.

It is tempting to think from these results that children, on account of their developing brains, may be particularly susceptible to the ill effects of household air pollution (HAP), cognition, or otherwise. Older adults exposed to these types of HAP, however, also show lower cognitive performance relative to those not exposed to household air pollutants.

Exposure to fumes from burning firewood or kerosene for cooking purposes was associated with a three-fold higher risk of developing cognitive impairment relative to individuals who were cooking using liquid petroleum gas. Given that most of this research stems from low- and middle-income countries, it is reasonable to assume that socio-demographic factors (e.g., rural rather than urban residence) may be driving these effects.

Large-scale studies such as the Longitudinal Ageing Study of India (LASI), however, have demonstrated that even after adjusting for socio-demographic factors, exposure to indoor air pollutants remains a significant contributor to poorer cognition. Saenz et al. (2018) found similar results in Mexican adults who used wood or coal as cooking fuel, even after controlling for a variety of demographic and health-related variables. Specifically, they found exposure to indoor air pollutants to negatively impact their performance on tasks of attention, fluency, and verbal learning. Longitudinal studies with older adults have also demonstrated a small, albeit significant, effect of unclean cooking fuel exposure on overall cognition.

Other sources of indoor air pollution and their relationship to cognition have also been investigated. Dampness, condensation in the house, and exposure to secondhand smoke were found to have a negative impact on children’s abilities. Exposure to either of these at 9 months and 3 years of age was observed to negatively impact verbal skills at 3 years of age, suggesting both a longitudinal and concurrent effect.

Even exposure to a single indoor pollutant such as incense burning, cooking oil fumes, home renovations, or secondhand smoke was associated with a higher incidence of ADHD-like symptoms in Chinese children. Lastly, a systematic review investigating the impact of indoor air pollution on adult cognition revealed almost all studies on this topic to have noted a positive relationship between exposure to air pollution and cognitive dysfunction, with elderly women found to be most susceptible to these effects.

In the context of low- and middle-income countries facing a dementia epidemic in years to come, and with the only reliable preventative measure being risk-modifying agents, the extent of indoor air pollution in this demographic is certainly concerning. The biggest and most widespread contributor to HAP in this demographic remains unclean fuel sources.

As per the National Family Health Survey, 41% of Indian homes continue to use solid fuels for cooking. Although the Indian government is working hard to try and address this through schemes such as the Pradhan Mantri Ujjwala Yojana (PMUY), current research indicates that this should be made a matter of priority. A simple change in terms of infrastructure might have lasting ramifications with regard to the long-term health of our population.

Prof. Deepa Bapat, Adjunct Faculty of Psychology, FLAME University:

Prof. Shweta Rana, Faculty of Environmental Studies, FLAME University:

Liz George, FLAME Alumna