Governments and scientists often judge air quality by the amount of fine particulate matter (PM2.5) in it. These particles can enter deep into the lungs and bloodstream and have toxic effects. New research from East China Normal University suggests, however, that this measure may not tell the whole story. Even when PM2.5 levels meet global safety standards, the air can still contain toxic particles that harm the lungs.
In the study, published recently in Environmental Health, scientists analysed PM2.5 levels in Shanghai in winter and spring 2025. Then they used a sensitive method called single-particle inductively coupled plasma time-of-flight mass spectrometry to identify the chemical makeup of each particle. They paid special attention to metal-containing fine particles (MCFPs), small fragments that include aluminium, silicon, iron, manganese, and lead.
The scientists found that MCFPs made up about 80% of all metal particles in the city’s air. They also used machine learning to determine the particles’ origins and tested how they affected human lung cells.
The results were striking. Air samples collected on clean days, when the PM2.5 concentration was under 15 µg/m3, were often more toxic to lung cells than those collected on polluted days. The oxidative stress in cells, a sign of tissue damage, was up to 8.1x higher and cell death was up to 6.3x higher, even as the total particle mass was lower.
The main culprit was iron-rich MCFPs that carried other toxic metals such as manganese and lead. These combinations triggered strong chemical reactions inside cells, releasing free radicals that damaged DNA.
The team found that most of these iron-rich particles came from human activities, especially vehicle emissions and burning coal. On days with heavy dust or haze, the MCFPs were partly masked by larger mineral particles, which reduced their relative abundance. But on seemingly cleaner days, the proportion of these particles was much higher, rendering the air chemically more dangerous.
The study concluded that PM2.5 mass alone is a poor indicator of air safety. Even air that seems clear can hide particles small enough to evade the body’s defences and persist in organs for years. The researchers suggested that air-quality monitoring and regulation should shift from measuring only total particle mass to identifying and controlling the specific toxic components, particularly iron-rich MCFPs from traffic and fuel combustion.