Season matters: Why Bangladesh’s dry months make toxic food risk worse

In winter, the vegetable fields around Savar look deceptively healthy. Rows of spinach, red amaranth and gourds stretch across the land, their leaves glossy and full, harvested daily for markets that feed Dhaka.

This is when production peaks, prices stabilise and households across Dhaka and all over the country consume the most fresh greens.

But beneath the soil, something else is peaking too.

Heavy metal contamination in agricultural land surrounding the Dhaka Export Processing Zone (DEPZ) intensifies during Bangladesh's dry months, particularly winter, turning the season of abundance into a period of significant food safety risk.

A December 2025 study published in Next Research, an international peer-reviewed multidisciplinary journal, shows that contamination levels of toxic metals such as cadmium (Cd), lead (Pb) and chromium (Cr) are consistently higher in the vegetable fields during the dry season.

The reason is not mysterious, but it is largely invisible: when the rains stop, polluted irrigation water and industrial dust quietly take their place.

"During the dry season, irrigation with industrial wastewater increases metal accumulation in soil and crops," said Nahid Mahmud, the study's lead author. "There is less leaching, more concentration, and greater retention of metals in surface soil."

The study synthesised findings from multiple national and international research efforts examining agricultural soils around DEPZ, one of Bangladesh's most industrialised zones.

Located on the outskirts of Dhaka, the zone houses textile, leather processing, metal finishing and electronics factories. Many of these factories discharge untreated or poorly treated effluents into canals that run directly alongside farmland.

These canals are not just drainage channels; in winter, they become lifelines for farmers. With rainfall nearly absent between November and March, irrigation water — often visibly dark and foul-smelling — is lifted from nearby canals onto crop fields.

Over time, this water deposits heavy metals into soil that has little opportunity to flush itself clean.

Seasonal dynamics matter because contamination is not static.

During the monsoon, heavy rainfall dilutes surface pollution and pushes some metals deeper into the soil profile or into adjacent water bodies.

"We have already seen findings from a UNICEF-supported survey indicating elevated levels of lead. According to that report, lead contamination was detected in around 34-38% of samples. Based on these findings, we are planning further action."

Zakaria, Chairman, Bangladesh Food Safety Authority

In winter, evapotranspiration increases, dust settles from the air, and metals remain concentrated in the topsoil — precisely where crop roots are most active.

"During the dry season, irrigation with industrial wastewater increases metal accumulation in soil and crops. During the monsoon season, heavy rainfall spreads contaminants over a wider area and redistributes metals within the soil profile. 

"So, metal build-up is higher in the dry season, while spread and mobility increase during the monsoon," Nahid said.

Among all crops, leafy vegetables emerge as the most vulnerable. Spinach, water spinach and red amaranth absorb and retain metals far more efficiently than grains or fruits. Rice, too, is a major concern because of its daily consumption and its ability to accumulate arsenic and cadmium when grown in contaminated soils.

"Leafy vegetables are the most exposed, but rice is the most dangerous from a food security perspective," Nahid said. "It is eaten every day by most households. Even small concentrations can add up."

The study uses multiple pollution indices — such as the Geo-Accumulation Index (Igeo), Pollution Load Index (PLI) and Ecological Risk Index (ERI) — to assess contamination severity.

Across indices, agricultural soils near DEPZ frequently fall into moderate to very high contamination categories. Mercury and arsenic consistently show extreme ecological risk in index-based assessments.

What makes winter particularly dangerous is timing.

This is peak season for vegetable cultivation, peak season for consumption, and peak season for contamination. The risks therefore multiply quietly.

Zakaria, chairman (additional secretary) of the Bangladesh Food Safety Authority, said they are aware of the food safety risks and planning a large-scale intervention.

"We have already seen findings from a Unicef-supported survey indicating elevated levels of lead. According to that report, lead contamination was detected in around 34-38% of samples. Based on these findings, we are planning further action," he added.

He said that with Unicef's support, they will conduct extensive food sample testing.

"Our aim is to identify where heavy metal concentrations are comparatively higher and to determine their sources. Initial reports suggest that lead is the dominant concern, while other heavy metals such as chromium appear in smaller quantities," Zakaria said.

"We are planning to test around 3,400 food samples. This will help us better understand the sources of contamination and guide long-term interventions. Addressing heavy metal contamination is part of our longer-term plan, spanning up to 10 years," he added.

The health implications of heavy metals in food are long-term.

Heavy metals enter the food chain slowly: soil to crop, crop to plate, plate to body. Over years, exposure can lead to kidney and liver damage, neurological disorders, bone diseases, developmental issues in children and increased cancer risks.

"Heavy metals act as silent toxins," Nahid said. "Their effects appear gradually, which is why people underestimate the danger."

Farmers, meanwhile, face a double burden. Contaminated soil reduces fertility and crop quality, while declining consumer trust threatens livelihoods.

For peri-urban farming communities around DEPZ, agriculture remains a primary income source despite the encroachment of industry.

The study argues that regulation alone is not enough unless it accounts for seasonality. Industrial effluent treatment plants (ETPs) may operate intermittently, monitoring is often irregular, and enforcement tends to focus on monsoon flooding rather than dry-season concentration. 

As a result, the months of highest contamination often receive the least scrutiny.

The DEPZ case is not unique, the researchers said. It reflects a broader dilemma in Bangladesh's development model: industrial growth advancing faster than environmental safeguards, with agriculture absorbing the external costs. 

Winter simply exposes this imbalance more sharply.