Critical Review of Lead Pollution in Bangladesh

Background. Lead (Pb) poses a severe threat to human health and the environment. Worldwide Pb production and consumption have significantly increased along with unplanned industrialization and urbanization, lead smelting, and lead-acid battery processing. The improper management of Pb-containing elements is responsible for Pb pollution. Lead's persistence in nature and bioaccumulation in the food chain can lead to adverse health impacts. Objectives. The present study aims to describe Pb contaminated sites in Bangladesh and Pb concentration in the atmosphere, water, sediments, soil, vegetables, fish, and other foods in Bangladesh. Methods. The present study searched a total of 128 peer-reviewed articles based on a predefined set of criteria (keywords, peer-reviewed journals, and indexing in Scopus, Science Direct, Web of Science, Springer, PubMed, Directory of Open Access Journals (DOAJ), and Bangladesh Journals Online (BanglaJOL) and exclusion criteria (predatory journal and absence of full text in English) and finally selected 63 articles (58 research articles and five (5) reports). The relevant findings on Pb exposure, sources, routes, diet, and impacts in Bangladesh were combined and presented. Results. The reviewed studies identified 175 Pb contaminated sites through soil sample assessment in Bangladesh. The study determined Pb concentrations in air (0.09–376.58 μg/m3, mean 21.31 μg/m3), river water (0.0009–18.7 mg/l, mean 1.07 mg/l), river sediments (4.9–69.75 mg/kg, mean 32.08 mg/kg), fish (0.018–30.8 mg/kg, mean 5.01 mg/kg), soil (7.3–445 mg/kg, mean 90.34 mg/kg), vegetables (0.2–22.09 mg/kg, mean 4.33 mg/kg) and diet items (0.001–413.9 mg/kg, mean 43.22 mg/kg) of which 38.8%, 27.8%, 54.5%, 68.8%, 9.7% and 100% of samples, respectively, exceeded related World Health Organization (WHO), Food and Agriculture Organization (FAO), United States Environmental Protection Agency (USEPA) and Bangladesh Standard Testing Institution (BSTI) guidelines. The present study found that industrial soils are severely polluted with Pb (7.3–445 mg/kg) in Bangladesh. A high Pb concentration has been found in fish muscle and foods, including leafy and non-leafy vegetables collected from different places in Bangladesh. Conclusions. Lead-contaminated foods can enter the human body through dietary intake and consequently lead to long-term adverse health effects. This study may help policymakers to formulate national policies with effective mitigation plans to combat the adverse health impacts of Pb in Bangladesh. Competing Interests. The authors declare no competing financial interests.


Introduction
The advancement of technology has led to the expansion of urbanization and industrialization, resulting in an upsurge of heavy metal pollution into the environment, especially in low-and middle-income countries (LMIC). Anthropogenic activities are a significant contributor to the amplification of heavy metal emissions into the earth's atmosphere, cultivated land, and water bodies. 1 Rapid and unplanned urbanization and industrialization have led to toxic substances like arsenic (As), cadmium (Cd), chromium (Cr), copper (Cu), mercury (Hg), zinc (Zn), nickel (Ni), tin (Sn), and lead (Pb) being discharged to the surrounding environment. 2 Planned urbanization also increases the risk of releasing toxic materials due to unregulated development activities. 3 These metals contaminate the air, soil, and water through various pathways, including vehicle exhaust, fossil fuel combustion, suspended atmospheric particles, untreated municipal sewage, fertilizer and pesticides, municipal solid waste, and mining activities. Suspended air particles hold heavy metals, which eventually deposit onto the soil through the natural sedimentation and precipitation processes. 1 Through various exposure routes such as diet, smoking, breathing, and drinking, toxic heavy metals can accumulate in various parts of the human body and cause severe health disorders. 3 Review biodegradable toxic heavy metal that is soft, corrosion resistant, highly malleable and ductile. As a result, Pb has been used in many industries. Lead has acute and chronic effects on human health and the environment. 4 Due to the rapid growth of industrial activities, the high demand for leadacid batteries (LAB) is now a concern in low-and middle-income countries. 3 Since the early days of the industrial revolution, use of Pb has increased globally (Figure 1), and over the last decade, the exponential growth rate has become much more significant. In the 20 th century, Pb contamination has increased significantly due to the use of leaded gasoline in motor vehicles. 5 The global usage of Pb in 2012 was 10.7 million tons 6 ( Figure  1). The demand for motor vehicles has led to increased LAB demand. 7 This Pb is 100 percent recyclable, but the process is conducted chiefly through informal processes, especially in developing countries, 8 resulting in soil contamination. The sector-based global annual consumption rate of Pb is shown in Table 1. Around 85% of Pb is used during the LAB manufacturing process. About 98.9% of used LAB (ULAB) were recycled in the United States in 2014, and around 99% in the European Union (EU) from 2010-2012. 6 Lead can have adverse environmental and health effects resulting from various anthropogenic activities.  Review μg/dL in recycling plants in low-and middle-income countries, which exceeds the current United States standard of 5 μg/dL. 30 Child laborers in low-and middle-income countries are at higher risk because of acute exposure to Pb as measured by blood Pb levels; levels exceeding 150 μg/ dL may cause death. 31,32 According to the Institute for Health Metrics and Evaluation (IHME) 2017, 33 Pb exposure was responsible for 1.06 million deaths worldwide, with the highest Pb-related death rate in lowand middle-income countries. Lead not only affects the human body, but also has an adverse impact on soil microbial communities and the growth of plants. 23 A summary of the effects of Pb poisoning on soil, plants, and humans is presented in Table 3.
Bangladesh is a highly populated and polluted country. 25 Along with rapid economic growth, it has experienced a dramatic shift in exposure to Pb over the past three decades. Bangladesh has a limited capacity for waste treatment and recycling facilities. Untreated wastes are discharged into nearby agricultural lands, rivers, roadside canals, or streams. Lead from these sources can persist in the soil and water bodies that humans can take up through the food chain. A previous study found that the deposition of Pb in soil, crops, water, air, and vegetables is higher in the vicinity of Bangladesh's industrial and urban areas. 3 Since the 20th century, two and three-stroke engines have been major contributors to atmospheric Pb pollution in Bangladesh. 5 There are 148 known informal recycling sites conducting ULAB and 97 LAB manufacturing sites in Bangladesh. This presents a threat to human health as people living near urban or industrial areas have higher blood Pb levels in their bodies. 8 The present study aims to determine potential Pb contaminated sites in Bangladesh and Pb concentrations in different environmental spheres.

Methods
A systematic literature search was conducted of research findings on Pb exposure from relevant sources such as peer-reviewed articles, textbooks, and reports of Pb contamination and poisoning in Bangladesh. The literature search focused on seven prioritized aspects of Pb pollution and pathways (atmosphere, water, sediment, fish, soil, vegetables, and foods) along with hotspots and impacts on living organisms. The search was performed through electronic databases using the following terms: "Pb in atmosphere, " "Pb pathway, " "Pb exposure, " "Pb in river water, " "trace metals in water, " "Pb in river sediment, " "Pb from industrial emission, " "heavy metal concentration in soil, " "Pb in food, " "Pb in vegetables, " "pathways and routes of Pb, " "Pb in the food chain, " "health effects of Pb, " "Pb effects on plants. " One hundred and twenty-eight (128) related research papers were identified from accepted publication platforms around the world (

Eligibility criteria
The following inclusion criteria were adopted: (i) peer-reviewed with mentioned database (ii) articles investigating Pb in the atmosphere, water, sediment, fish, soil, vegetables, and foods; (iii) full texts published in English; (iv) use of scientific analytical methods; (v) discussion and interpretation of the main findings;  (vi) limitations of the study (e.g. process of data collection, lack of data, lack of ethics clearance, lack of coherence in data analysis) and (vii) conclusions and implications within the scope of the study design. There were no restrictions on the date of publication. Criteria for exclusion were: (i) publication in predatory journals (predatory journals defined as not indexed, rapid publication process, contact email address is nonprofessional and non-journal affiliated, peer review process and publication timelines too short etc.), and websites (ii) published papers with English abstracts but without full texts in English.

Data processing
We followed two screening (abstract and full text) procedures on retrieved literature to determine article eligibility based on this study's objective. During the first screening, the title and abstract were reviewed based on Pb sources, route and impacts in Bangladesh. Next, the full text of those related abstracts was assessed to identify whether a study was fully or partially related to the study's objectives. Finally, 63 studies were selected for review. An overview of the literature selection procedure is shown in Figure 2. Lastly, findings were processed and analyzed following the cross-tabulation technique to list and compare Pb concentrations from various sources. For hotspot mapping and tabulation, ArcGIS 10.2.1 and Microsoft Excel 10 were used, respectively. Table 4 presents the characteristics of the included papers. We considered 58 research articles from seven different media. In addition, five studies were examined for the standard concentration of various spheres. Two (2) of the 58 reviewed articles' key objectives was to classify the sources of heavy metal pollution, 48 articles described metal concentration, and six described health risks from Pb exposure. Lead concentrations in soil was the focus of 22 (32%) studies, and water and sediment results were reported in 12 papers (16%). Most of the first authors of these peer-reviewed articles were from Bangladesh, followed by the United States.

Results
In collaboration with the Department of Geology of the University of Dhaka and the Department of Environment, Bangladesh, Pure Earth investigated Pb contaminated hotspot areas in Bangladesh. The investigators collected soil samples, following the Initial Site Screening (ISS) protocol provided by Pure Earth, 43 and determined the hotspot zones based on the analyzed results. One hundred and seventyfive (175) of the assessed sites were found to be contaminated with Pb. 43 Eighty-five (85) battery recycling/ manufacturing/repairing and 84 Pb smelting industries were identified as sources of Pb pollution in Bangladesh. In addition, tannery and dye operations, heavy industry, chemical, and fertilizer manufacturing were identified as sources of Pb pollution in different districts of Bangladesh. Figure 3 shows the spatial distribution of Pb-contaminated hotspot areas in Bangladesh. Dhaka and Khulna districts were found to be the most polluted and have more smelting and ULAB industries than other districts.

Discussion
After the banning of two and threestroke motor vehicles in 2002, the most significant Pb emission sources in Bangladesh are now industries that use substances containing Pb, mostly informal ULAB, and the smelting industry. Figure 3 shows that most of the contaminated sites are either ULAB or smelters. Heavy traffic congested areas and highly industrialized zones in Dhaka and Chittagong had atmospheric Pb levels exceeding the Bangladesh standard of 0.5 µg /m 3 . 47 In Chittagong, the New Market and Director's office areas are two of the city's busiest areas, and road dust and vehicle exhaust were the primary sources of Pb pollution in Chittagong. 78 Near a battery manufacturing plant in Munsiganj Sadar Upazila, including the surrounding residential areas, the concentration was higher than other locations.
Being a river-fed country, resources from rivers help the country's economic growth, but contaminated and polluted resources like water, sediment, and fish can harm humans through the food chain and biomagnification process. Data were compiled into a frequency table (Table 12) to determine the most contaminated sector. Based on the percentage of contaminated samples, the most polluted sector was vegetables, where 100% of the studied samples were found to be contaminated with Pb followed by 68.8% of the fish species. Soil was found to be the least contaminated sector in Bangladesh. The authors did not find any direct relationship in Pb concentration between soil and associated vegetables or foodstuff. Since all the spheres of the environment are interrelated, there is a need for more research on Pb concentration in air, water, soil, and foodstuff grown in specific areas.
Comparing the average concentration with the standards of each sector, the most polluted sector was found to be vegetables.
Several studies have reported that Bangladeshi populations are exposed to excessively high levels of Pb in their diet and through inhalation.

Study limitations
Documents published in languages rather than English were excluded, and we focused on papers that were available online only, thus potentially excluding some useful sources. Another limitation of the study was the heterogeneity in the sample collection procedures in included papers. Despite these limitations, the present study can provide an overview of Pb pollution in Bangladesh.