In 2020, Dhaka, Bangladesh ranked as the second most polluted capital city globally. Its nearly 22 million residents were exposed to an annual average PM2.5 concentration of 77.1 μg/m3, more than 7 times higher than the World Health Organization (WHO) annual mean target for safer air at 10 μg/m3.
Dhaka’s chronically polluted skies are a major public health concern. The WHO estimates that outdoor air pollution kills an estimated 108,000 Bangladesh residents every year, while countless others suffer morbidities associated with long-term exposure.1
The COVID-19 pandemic has brought a renewed focus on the health impacts of PM2.5 pollution. A respiratory illness, COVID-19 disproportionately affects populations exposed to long-term air pollution.2 In South Asia, an estimated 15 percent of COVID-19 deaths are attributed to air pollution exposure.
By some measures, the early months of 2021 have been the most polluted on record. At the same time, Dhaka has suffered its highest number of COVID-19 deaths since the pandemic began, despite vaccination rollout across the country.3
This analysis examines the reasons behind Dhaka's relatively high wintertime pollution levels and presents the status of air quality in the capital during the early months of 2021, including its peak as the most polluted city in the world (see Figure 1). The consequences of this record-breaking air pollution will likely play out during the rest of the year and beyond.Figure 1: On April 23rd, 2021 Dhaka topped the Most Polluted Cities ranking. Source: IQAir' src="https://cms.iqair.com/sites/default/files/inline-images/Dhaka-most-pollution.png">
Importance of PM2.5
PM2.5, or particulate matter measuring 2.5 micrometers in diameter or smaller, describes a mixture of microscopic solid and liquid particles, including:
- smoke or soot
- wind-blown salt
PM2.5 is frequently regarded as the most dangerous air pollutant to human health. Due to its microscopic size, it is able to penetrate deep into the lungs and into the bloodstream. Once transported in the blood, PM2.5 can affect a number of organs and contribute to a range of morbidities, including:
- chronic obstructive pulmonary disease (COPD)
- respiratory infections
- heart attack
PM2.5 emission sources
Data used in this study reveals that while Dhaka’s annual PM2.5 exposure has been on the decline since its peak in 2018 at 97.1 μg/m3, 2021 may prove to be an exception.
Dhaka ranks third among the fastest growing megacities worldwide.4 On average, 536,000 new residents move to the city every year, usually migrating from the countryside for better employment opportunities.
Much of the city’s PM2.5 emissions are linked to its rapid population growth, with brick kilns and dust from construction sites contributing 73 percent of citywide PM2.5.5 Motor vehicles are the next largest source, comprising 10.4 percent of total PM2.5 emissions, followed by lead, biomass burning, and sea salt.
Brick kilns are the single largest source of PM2.5 pollution in Dhaka, Bangladesh. Over 2,000 kilns exist in the capital, burning coal to create bricks for new construction. Many are operating illegally.6
Brick kiln chimneys release dense plumes of PM2.5 smoke as well as:
While no government authority has been designated to control air pollution in Bangladesh or in Dhaka, local and federal policy has been established to reduce emissions from brick kilns, including:7
- requiring fixed-chimney kilns be converted to zig zag kilns capable of reducing emissions by more than 50 percent
- imposed emission standards on kilns and fines for exceedances
- ending environmental clearance certificates on brick kilns
- promoting the transition from brick to concrete blocks by 2025
Dust is the second leading cause of PM2.5 pollution in the capital.
Located on a river delta, Bangladesh is built on sand and fine sediment that is easily kicked up and resuspended, particularly during construction when ground is broken for new development.
Construction on Dhaka’s Metro Rail Project and Elevated Expressway Project are expected to be major sources of dust resuspension in the coming years.8
In Dhaka, fossil-fuel-powered vehicles are the third leading source of PM2.5, comprising 10.4 percent of all PM2.5 emissions.
As the capital’s population has grown, so too have the number of vehicles. Between 2010 and 2018, Dhaka has observed a 68 percent increase in the number of vehicles on the road as the prevalence of certain types of vehicles (e.g., trucks, jeeps, minivans) more than doubled.
Dhaka’s seasonal air pollution
Dhaka experiences strong seasonal deviations in air quality. December through March are the most polluted months of the year, often experiencing PM2.5 concentration levels more than six times higher than summer months.
Elevated wintertime pollution is the result of both increased emissions as well as weather conditions that prevent pollution dispersion.9
Other causes of increased PM2.5 emissions during winter include:
- soil erosion from tilling and less frequent rainfall in the winter dry season
- seasonal operation of 1,000 brick kilns as construction picks up during cooler and drier winter months
- road and soil dust emissions from increased construction activity
Winter weather conditions that allow particle pollution to accumulate and linger include:
- low wind speed
- infrequent rainfall, which normally helps to coagulate or tamp down PM2.5 and other pollutants
- transboundary air pollution from India to Dhaka through the Indo-Gangetic Plain (IGP) corridor
- temperature inversions that trap air pollutants close to the ground and prevent normal dispersal
2021 Dhaka Pollution Findings
During the first three months of 2021, Dhaka averaged a PM2.5 concentration of 154.6 μg/m3.
Compared to the same time period in years past, the start of 2021 was more polluted than the start of 2020 (141.8 μg/m3), 2019 (144.8 μg/m3), and 2017 (146.6 μg/m3), but less polluted than the start of 2018 (156.3 μg/m3).
While the three-month monitoring period in 2021 was 1.1 percent less polluted than 2018 (the most polluted year on record), data indicates that 2021 will be more polluted than 2018 and any other year in recorded history for Dhaka.
2021 had the most polluted March on record
March 2021 averaged a PM2.5 concentration of 118.0 μg/m3, higher than any other March in the last 5 years or since air pollution records began for Dhaka. March 2018 ranks as the second most polluted March in the 5-year span, with a level 7 percent lower than March 2021 (110.0 μg/m3).
Every day in March 2021 averaged a U.S. AQI “unhealthy” PM2.5 concentration or worse, which has never before happened in the 5-year monitoring period (see Figure 2).Figure 2: Average annual and monthly PM2.5 concentrations in Dhaka, color-coded by U.S. Air Quality Index categories. Source: IQAir" src="https://cms.iqair.com/sites/default/files/inline-images/Dhaka-monthly-averages-v2.png">
No hours met U.S. AQI standards for “good”
During the first three months of the year, no hours in Dhaka met the U.S. Air Quality Index (AQI) standard for “good.”
Only 2.3 percent of hours were deemed “moderate” or “unhealthy for sensitive groups,” while the remaining 97.7 percent of hours were classified as “unhealthy” or worse (see Figure 3).Figure 3: Dhaka’s distribution of hourly PM2.5 concentrations from January 1 to March 31, shown as U.S. Air Quality Index categories. Source: IQAir" src="https://cms.iqair.com/sites/default/files/inline-images/Dhaka-percent-hours.png">
2021 had the most hours with a concentration above 150.4 μg/m3, “very unhealthy”
During the first three months of 2021, 47.1 percent of hours were deemed “very unhealthy” or worse – the most of any year in recorded history.
By comparison, the number of hours that averaged a “very unhealthy” concentration (≥ 150.4 μg/m3) in years past are (see Figure 4):
- 2020: 40.2 percent
- 2019: 40.6 percent
- 2018: 44.4 percent
- 2017: 42.0 percent
Air quality in Dhaka is an environmental risk for heart and lung disease and early death.
Since 2017, the beginning of Dhaka’s air quality data records, every year has averaged pollution levels more than 7 times the WHO recommendation, falling into the U.S. AQI “unhealthy” range.
Early data indicates that 2021 may be Dhaka’s most polluted year on record. For the first three months of the year, 2021 lagged behind 2018 (the most polluted year) by just 1.1 percent. But 2021 has already set records, with the most polluted March in Dhaka history and the highest number of hours meeting the “unhealthy” U.S. AQI designation.
To reduce annual average PM2.5 emissions in the capital, focus should be placed on winter emissions.
Regulations that control dust from construction sites and brick kiln activity could also be better mitigated with policy and enforcement. Transitioning from bricks to concrete block alternatives as well as from fossil fuels to lower-emission or renewable sources of energy could both significantly reduce PM2.5 in Dhaka.