Spontaneous Combustion Explained: From Linseed Rag Hazards to Lithium Battery Dangers

In this Curious Cases episode, scientists dissect spontaneous combustion, explaining how materials like linseed-oil-soaked rags can heat up and ignite without an obvious spark. The discussion expands to everyday risks, including grass piles, paint tins, and especially lithium batteries, which can undergo thermal runaway when damaged or misused. Fire investigators weigh in on real-world ignition scenarios, while a forensic pathologist debunks the idea of spontaneous human combustion as a widespread medical event. The show also explores practical safety measures for handling rags, batteries, and household devices to reduce fire risk.

Introduction and framing: turning a sensational topic into science

The episode begins with host Hannah Fry acknowledging that listeners bring quirky questions to Curious Cases, with spontaneous combustion as a particularly provocative example. The hosts acknowledge that mysteries from the 1970s television era are less relevant today, given the ubiquity of cameras and modern materials, but the question remains: what actually causes spontaneous ignition in everyday life and in modern technology? The show promises a careful, science-grounded examination of a topic often surrounded by sensational headlines, while also leaving room for some lighter demonstrations that illustrate chemical principles in action.

What spontaneous combustion actually is: the chemistry explained

Andrew Sala, a professor of inorganic chemistry, outlines the basic chemistry behind combustion: a fuel reacting with an oxidizer in an exothermic process that releases heat and light. The “spontaneous” aspect is the self-acceleration of this process when a material warms up slowly enough to cross a temperature threshold, after which the reaction becomes self-sustaining. This requires three ingredients: a fuel, an oxidizer, and heat. In conventional fires you usually have an ignition event, but spontaneous combustion can occur when one of the ingredients and the environment lead a material to heat up gradually until it crosses the fire threshold.

"Spontaneous combustion is not magic; it is a chemical process driven by heat, fuel, and oxidizer that can reach a self-sustaining threshold" - Andrew Sala, Professor of Inorganic Chemistry, University College London

From storage to real life: linseed oil rags and other everyday hazards

The conversation turns to linseed oil soaked rag storage, a classic example of autoxidation. Linseed oil, a polyunsaturated compound, reacts with air slowly. When spread thinly on a rag, the surface area is large enough to draw in oxygen efficiently, and the heat generated by the oxidation can accumulate. If the rag is left in contact with other fuels or in a poorly ventilated space, the heat can accumulate and reach a melt-ignite threshold. The discussion also touches on other common everyday risks, including the heating of grass and hay under hot summer conditions, particularly when fermentation and bacterial activity generate additional heat. These cases illustrate how a combination of fuel, oxygen, and heat can lead to ignition without a spark, though not all such scenarios result in a blaze, depending on the surrounding environment and material properties.

"Linseed oil reacts with air very gently, and the rag can warm up and reach a threshold where it takes off" - Andrew Sala

Modern risks: batteries and electronic devices in the daily life of a battery-reliant society

The episode then investigates the modern-day risk landscape created by lithium batteries. Matt Oakley explains that lithium metal itself is highly reactive and flammable, and the battery architecture has multiple components that can fail in ways that trigger thermal runaways. Short circuits, improper charging, and physical damage can all undermine the safety mechanisms inside a battery, causing heat to accumulate and propagate. The host notes the rising incidence of battery-related fires in vehicles, consumer electronics, and storage scenarios, underscoring how everyday devices can become ignition sources if mishandled. The discussion extends to how these fires can be difficult to control because of the rapid energy release and the possibility of many batteries igniting in succession.

"This reaction is exothermic, heat energy is released, and thermal runaway can occur when lithium batteries are damaged or charged improperly" - Matt Oakley, Fire Investigation Officer

Real-world ignition: what fire investigators see and how they reason through causes

Matt Oakley shares the practical realities of fire investigation, noting that fires have multiple potential ignition sources and that investigators use systematic elimination to identify credible hypotheses. The discussion emphasizes that the presence of a battery fire in a vehicle or storage area can be the dominant ignition source, and that the ignition can spread across compartments rapidly once a critical temperature is achieved. The interview reveals a growing database tracking lithium battery fires, and it conveys the complexity of determining ignition sequences in real-world scenes where multiple heat sources may be involved.

"We investigate all possible hypotheses and eliminate the non-credible ones to identify the most plausible cause" - Matt Oakley

Spontaneous human combustion: debunking a persistent urban myth

Forensic pathologist Roger Byard discusses spontaneous human combustion, tracing the idea across centuries and noting how it has appeared in literature and folklore. He explains that the canonical cases involve an elderly, obese person confined to a space, often with blankets and alcohol or cigarette use, and that the body shows partial burning consistent with a slow, smoldering fire rather than a sudden explosion. The crucial point is that spontaneous combustion, as a medical event where a person suddenly bursts into flames, has never been observed in real time. In practice, what is documented is a smoldering fire that burns through fat tissue, sometimes leaving only limbs and ash, with the surrounding environment largely intact. The segment emphasizes that the scientific explanation aligns with a smoldering, contained fire rather than any supernatural mechanism.

"Spontaneous human combustion is basically a long-standing urban myth; cases show smoldering fires contained by blankets rather than a sudden ignition" - Roger Byard, Professor of Pathology

Safe practices and the big takeaway for a battery-powered world

The show closes with practical advice on reducing fire risk. Handling oily rags requires safe storage away from heat sources and other fuels, and disposing of or washing rags appropriately after use. Batteries, particularly lithium-based cells, demand proper charging equipment, damage assessment, and safe storage, especially for devices kept in shared spaces or in transit. The conversation reiterates that batteries are not inherently dangerous, but irresponsible handling and storage are the key risk factors. The panel reflects on how the modern reliance on batteries changes the risk landscape, urging audiences to adopt responsible charging practices and environmental awareness as part of everyday safety.

"Keep rags separate, store them away from other fuels, and use the correct charger for lithium batteries to reduce fire risk" - Matt Oakley