Almost every morning this past winter, media outlets reported on Dhaka's poor air quality.

Of the first 53 days of the year, Dhaka had the worst air quality in the world on 19 days. In fact, Dhaka's poor air quality means it has been outside of the top five on only five days this year.

A worldwide ranking of cities with the worst  air quality based on air quality index (AQI) is available online.

The AQI is a way of measuring air quality in a specific location based on the presence and intensity of pollutants in the air.

The AQI in Bangladesh is based on five pollutants: particulate matter (PM10 and PM2.5), NO2, CO, SO2, and ozone. Scores on the AQI scale get higher the worse the air quality gets in a given area or city. Scores between 51 and 100 signify moderate air quality, 101-150 is said to be unhealthy for sensitive individuals, 151-200 is unhealthy for everyone, 201-300 very unhealthy, and above 300 hazardous.

In the last 30 days, Dhaka's average daily AQI has been above 150 every day, according to iqair.com. It was between 201-300 on 18 occasions, and on January 25, Dhaka's average AQI was 317.

So, Dhaka's air quality is at the very least unhealthy and at times hazardous, but does high AQI lead directly to bad health?

Although most research on this is still inconclusive, and it is currently an open area in the fields of environmental and medical science, recent research conducted in countries like India and Thailand shed some light on the issue.

An article by Indian researchers published in Nature in November, 2023 examined whether AQI and prevalence of lung cancer were correlated. This research showed a "significant positive correlation between high AQI levels and a higher rate of lung cancer", but did not prove causality.

In Thailand, researchers studied over 3,500 hospital emergency room visits for heart and respiratory health issues in the span of a year.

Research was conducted to find a correlation between these emergency room visits and the AQI of the air these individuals were exposed to in the days leading up to the emergency room visits. This research -- published in November last year in peer-reviewed journal PLOS ONE -- demonstrated "the association between short-term PM2.5 pollution exposure and increased risks of emergency room visits for acute coronary syndrome and respiratory diseases, particularly pneumonia".

PM10 exposure was similarly associated with chronic obstructive pulmonary disease, asthma, as well as high rates of hospitalisation for heart failure.

While correlation between high AQI and heightened risk of lung and heart disease has been established, scientists are yet to definitively prove causality.

People who are subjected to Dhaka air, however, have often complained of the effects of pollution.

Anupoma Joyeeta Joyee, a lawyer who lives in the capital's Uttara, has been an asthma patient since childhood. Every winter, her respiratory health declines, and this year she minimised her outdoor activities.

"I have to cross the Khalpar area in Uttara every day if I go outside, so I have tried to work from home as much as possible recently. I still have to go out once in a while, and every time I do, my breathing is affected. I try to make sure I'm wearing a mask whenever I'm outside, but I'm still in a panicked state because I know that not all pollutants are being blocked by it," she said.

Syeda Maisha Ali, a private university teacher, told The Daily Star about her father who was taken ill following prolonged exposure to dust and other pollutants during the winter.

"My father has to travel for work, so during November and December, he used long-haul public transport numerous times. He is an asthma patient and he has been ill with throat and windpipe-related complications for almost a month," she said.

Speaking about how worsened air quality in winter may have impacted patients, Md Rahmatullah Khan, an associate professor at the Institute of Health Technology and a chest diseases specialist and pulmonologist, said, "Asthmatic patients have increased, there has been a high prevalence of acute exacerbation of asthma too. I have been seeing patients of all ages suffer from these problems.

"We advise patients to avoid allergens, to stay away from dust and smog. They should use warm water for showers and ablutions, and wear masks outside at all times," he added.