Monte Carlo-Based Probabilistic Health Risk Assessment of Airborne Heavy Metals from a Medical Waste Incinerator in Southwestern Nigeria

In many parts of the world, hospitals generate a unique and dangerous kind of trash: used bandages, contaminated syringes, and other materials that can carry disease. To get rid of this waste safely, many facilities burn it. While this process destroys harmful germs, it can also release invisible particles into the air, including tiny amounts of heavy metals like lead and mercury. These metals are not just dust; they are toxic elements that can settle on the ground or float in the air, eventually entering the lungs of people living or working nearby. For decades, scientists have tried to measure the danger these emissions pose to human health. Traditionally, they have used a method that relies on single, fixed numbers to guess the risk, assuming that everyone breathes the same amount of air for the same amount of time. However, real life is rarely so simple. People move, the wind changes, and pollution levels fluctuate. To truly understand the danger, experts need a way to account for this natural variation and uncertainty, moving beyond a single guess to see the full range of possible outcomes.

In a recent study focused on a medical waste incinerator in southwestern Nigeria, researchers set out to see how much danger these emissions pose to the surrounding community. They chose the Federal Teaching Hospital in Ido-Ekiti, where a large incinerator burns thousands of kilograms of medical waste. Instead of just taking a single snapshot of the air, the team collected dust that had settled on glass slides placed at various distances from the burning unit, ranging from just ten meters away to several hundred meters downwind. They gathered these samples over two months, capturing the dust that drifted from the smoke. Back in the laboratory, they analyzed the slides to identify and measure ten different heavy metals trapped in the dust, including cadmium, chromium, and arsenic. The goal was not just to list what was there, but to understand how likely it was that a person breathing that air would face serious health problems.

To get a clearer picture of the risk, the team moved away from the old method of using fixed numbers. Instead, they used a powerful computer technique called a Monte Carlo simulation. Imagine trying to predict the weather for a whole year; you cannot just pick one temperature and assume it stays the same. You have to run thousands of scenarios, changing the wind, the humidity, and the time people spend outside, to see what usually happens and what extreme events might occur. The researchers did something similar for health risk. They fed their computer model with the actual metal concentrations they found, but they also let the other factors vary. They told the computer to simulate thousands of different people, some spending eight hours a day near the incinerator, others twenty-four, and some living there for ten years, others for fifty. By running ten thousand of these scenarios, they could see the full spectrum of risk, from the most likely outcome to the worst-case scenarios that a simple calculation would miss.

The results were stark and revealed a level of danger that the traditional methods would have hidden. When the researchers looked at the risk of non-cancerous health effects, such as damage to the lungs or kidneys, they found that in nearly every single simulated scenario, the danger was far above the safety limits set by international health agencies. The typical risk level they calculated was hundreds of times higher than what is considered safe. In fact, the probability of the risk exceeding the safety threshold was essentially one hundred percent. The study showed that the most common risk level was around four hundred times the safe limit, with some extreme scenarios reaching over two thousand times the limit. This means that for anyone breathing the air near this incinerator, the chance of suffering health harm is not a small possibility; it is a near certainty.

The danger was even more severe when looking at the risk of developing cancer over a lifetime. The simulations showed that the chance of getting cancer from breathing this air was also one hundred percent above the acceptable safety benchmarks. Even the lowest risk estimate in their thousands of simulations was far higher than what regulators consider a safe level for the general public. The researchers identified that the main driver of this high risk was simply the concentration of the heavy metals in the air, combined with how long people were exposed to it. The more time a person spent near the incinerator, and the higher the metal concentration, the greater the danger. The study confirmed that the current setup, with no significant buffer zone between the burning unit and the surrounding areas, is creating a hazardous environment.

This research highlights a critical gap in how we assess environmental safety in many developing regions. By relying on simple, fixed estimates, authorities might have missed the severity of the threat, thinking the risk was manageable when it was actually extreme. The study concludes that the current situation requires immediate attention. It suggests that hospitals need to create strict safety zones where people are kept far away from the smoke, and that the incinerators themselves need to be upgraded with better technology to filter out the toxic particles before they escape. Without these changes, the people living and working near these facilities remain at significant risk of serious illness. The findings serve as a clear warning that when it comes to toxic air, the worst-case scenarios are not rare exceptions; they are the reality for those living in the shadow of uncontrolled medical waste burning.