Hong Kong’s Centre for Food Safety has clarified that the unsettling blue-green residue found on scallions is a copper-based agricultural product, not a toxic additive—but consumers are urged to adopt simple inspection habits.
A wave of distress has swept through local online communities as shoppers report discovering a strange blue-green pigment on freshly bought scallions. From wet market stalls to supermarket shelves, concerned residents have taken to social media with photos and videos showing tissue paper turning an alarming shade of teal after wiping the vegetable’s leaves. Many describe the colour as persistent, surviving multiple washes and leaving them questioning whether their cooking ingredients are safe to eat.
The issue first gained traction when a Threads user shared their experience, and the post quickly went viral. Hundreds of commenters echoed similar stories, with many claiming that “every stall at the market” sells affected scallions. The uproar reached such a scale that local legislator Steven Ho Chun-yin formally wrote to the Centre for Food Safety (CFS), urging officials to address public anxiety and clarify whether these vegetables pose a health risk.
What Causes the Blue-Green Colour?
According to the CFS, the phenomenon is linked to Bordeaux mixture, a water-soluble copper compound fungicide widely used by farmers to protect crops from fungal infections. The copper ions in this agricultural product leave behind a characteristic blue-green stain on the leaves. This explanation aligns with expert commentary from mainland China, where the substance is commonly referred to as “blue alum” (藍礬), a form of copper sulphate.
Dr. Li Ming, a medical doctor from Peking University Cancer Hospital, explained on a mainland television programme that copper sulphate serves as an industrial preservative and insecticide. When sprayed on vegetables, it helps maintain freshness and prevents leaves from turning yellow by replacing magnesium ions in chlorophyll with copper ions—effectively keeping produce looking vibrant for longer periods.
Is Consumption Dangerous?
Health experts have weighed in to ease public concerns, though they acknowledge legitimate reasons for caution. Professor Rong Li from the School of Chemistry at Xihua University noted that if copper ions enter internal organs in high concentrations, they could cause damage. However, Professor Zhao Jing from the College of Food Science and Nutritional Engineering at China Agricultural University offered a more reassuring perspective.
“Copper is classified as a heavy metal, but it is also an essential trace element for the human body,” Zhao said. “Copper sulphate is considered a relatively safe pesticide, and copper ions do not accumulate toxicity in the body. The likelihood of poisoning from residual Bordeaux mixture is extremely low.”
She recommended that anyone who purchases scallions with visible blue-green residue simply rinse them under running water two to three times. For extra peace of mind, consumers can also cut away the most affected sections before cooking.
Three Expert-Backed Checks for Safe Scallions
To help shoppers make informed decisions at the point of purchase, Sun Shuxia, a consultant for the World Health Organization’s Health Education and Promotion Research Centre, shared three straightforward methods for identifying potentially treated scallions:
- Look (觀察顏色): Examine whether the green colour appears unnaturally intense. Check the white bulb section for any unevenly distributed blue-green spots.
- Touch (測試甩色): Gently rub the leaf surface with your fingers. If a blue substance transfers onto your skin or if the colour appears to rub off, this may indicate treatment.
- Smell (辨別氣味): Bring the scallions close to your nose. Treated vegetables often carry a faint metallic or rust-like odour.
Broader Implications and Consumer Awareness
While the current scientific consensus suggests that trace amounts of copper sulphate residue are unlikely to cause immediate harm, the incident highlights a growing demand for transparency in Hong Kong’s food supply chain. Legislative attention, combined with active citizen reporting, has already prompted the CFS to reiterate its official guidance on safe handling practices for fresh produce.
For concerned consumers, the key takeaways are clear: thorough washing remains the most effective defence, and visual inspection before purchase can help avoid the issue altogether. As the conversation continues to evolve online, food safety authorities are expected to monitor the situation closely and may issue further guidance should new evidence emerge.
In the meantime, home cooks can take comfort in knowing that a simple rinse—not panic—is the recommended response to these blue-tinted scallions.