Mechanistic Pathways of Biochemical and Toxicological Responses to Waste Tyre Combustion Emissions: A Systematic Review

Every year, the world generates millions of discarded tires. These are not ordinary trash items; they are engineered to be tough, resisting wear and tear for decades. When they finally reach the end of their life, they pose a significant disposal challenge. In many places, the solution has been to burn them. Whether it is a massive fire at a tire storage yard, a worker using a burning tire to singe hair off an animal hide, or a butcher using the same method to prepare meat, the result is the same: a thick, black plume of smoke rises into the air. This smoke is not just soot; it is a complex cocktail of chemicals released when rubber, steel, and synthetic additives are subjected to intense heat. For people living near these fires or working with them, the question is no longer just about the smell, but about what happens inside their bodies when they breathe this air or eat food prepared this way.

Scientists have long known that burning materials like rubber releases harmful substances, including tiny particles that can lodge deep in the lungs and chemical compounds that can damage cells. However, the specific chain of events that turns this smoke into illness has remained somewhat unclear. A new systematic review, led by researchers from institutions in Ghana and Nigeria, sought to piece together the missing links. The team did not conduct new experiments themselves. Instead, they acted as detectives of existing science, gathering and analyzing ten specific studies published between 2016 and 2025. These studies ranged from experiments on rats and mice to observations of human workers like firefighters and butchers, and even investigations into communities living near tire fires. Their goal was to map out exactly how the body reacts to these emissions, from the moment the chemicals enter the system to the damage they cause in specific organs.

The researchers found that the body's reaction to tire smoke follows a distinct and damaging pattern. It begins with a phenomenon known as oxidative stress. Imagine the body's cells as delicate structures that rely on a balance of energy and protection. When toxic chemicals from burning tires enter the body, they trigger an overproduction of unstable molecules called free radicals. These molecules act like tiny, uncontrolled sparks that scramble through the cells, damaging their walls and internal machinery. The body tries to fight back with its own natural defenses, antioxidants, but the review found that the sheer volume of these sparks often overwhelms the defenses. In the studies where researchers measured these levels, they saw a clear drop in the body's protective antioxidants and a corresponding rise in damaged fats within the cells, a process known as lipid peroxidation. This is the first critical step where the smoke begins to physically harm the body's tissues.

Once this internal damage starts, it triggers a second wave of response: inflammation. Just as a cut on the skin causes redness and swelling as the body tries to heal, the internal damage caused by tire smoke causes the body to launch a systemic inflammatory response. The review highlighted that this is not just a minor irritation. In human workers exposed to tire smoke, blood tests showed elevated levels of C-reactive protein, a well-known marker that signals the body is under significant stress. In animal studies, researchers observed actual inflammation in the liver and kidneys, with immune cells flooding into these organs. This inflammation, while a natural defense, becomes harmful when it persists, contributing to the breakdown of healthy tissue and setting the stage for long-term organ failure.

The review identified three main organs that bear the brunt of this assault: the lungs, the liver, and the kidneys. The lungs are the first line of defense because the smoke is inhaled. Studies showed that exposure leads to structural damage, including the destruction of the tiny air sacs where oxygen exchange happens and thickening of the blood vessels within the lung tissue. In human workers, this translated to measurable declines in lung function and a higher frequency of coughing and breathing difficulties. The liver and kidneys, which act as the body's filtration and detoxification systems, also suffered. Because these organs are responsible for processing and removing toxins, they become concentrated targets for the chemicals in the smoke. The review found consistent evidence of liver injury, marked by the leakage of enzymes that usually stay inside liver cells, and kidney damage, indicated by rising levels of waste products like urea and creatinine in the blood. In the most severe cases observed in animal studies, this led to cell death and scarring within these vital organs.

While the lungs, liver, and kidneys were the most clearly affected, the researchers also found hints that the damage extends further. Some studies suggested that the heart muscle could be injured, showing signs of stress similar to a mild heart attack, and that the body's hormonal balance, particularly the hormones controlling reproduction, could be disrupted. There was also evidence that the smoke affects the blood itself, potentially reducing the number of red blood cells. Crucially, the review confirmed that these chemicals do not just stay in the air; they enter the body. Biomonitoring studies of firefighters and butchers showed that their urine contained high levels of the specific breakdown products of the chemicals found in tire smoke, proving that the body absorbs these toxins and circulates them throughout the system.

The authors of the review were careful to note the limits of what they could conclude. The evidence base is still small, consisting of only ten studies, and many of the most detailed findings came from animal experiments rather than long-term human observation. This means that while the biological pathway from smoke to organ damage is strongly suggested, the full extent of the risk for humans over a lifetime is not yet fully mapped. The studies also varied widely in how they were conducted, making it difficult to combine their results into a single, precise number. However, the consistency of the findings across different types of studies—from the lab to the workplace to the community—provides a compelling picture. The review concludes that waste tire combustion is a significant, yet often overlooked, health hazard. It is not merely an environmental nuisance but a source of toxic emissions that can trigger a cascade of cellular damage, inflammation, and organ injury. The findings call for better waste management practices and a deeper understanding of the risks faced by those who live and work near these fires, urging a shift away from burning tires toward safer, more sustainable disposal methods.