Ep.30: Spontaneously Combustible Substances: The Fire Risk You Didn't See Coming

Some chemicals don’t need a flame to catch fire — just exposure to air. 

In this episode, Patrick Gee unpacks the hidden dangers of Class 4.2 spontaneously combustible substances — and what it takes to store and handle them safely. 

You’ll learn: 

• The difference between pyrophoric and self-heating substances 
• How Packing Groups classify danger levels 
• The strict cabinet requirements under AS/NZS 5026 
• What signage and ventilation systems are legally required 
• Why training and risk assessments are non-negotiable 

Whether it’s white phosphorus or aluminium alkyls, this episode breaks down how to control one of the most underestimated fire risks in the workplace. 

🎧 Subscribe now to keep your dangerous goods knowledge sharp and your team safe. 

Chapters

• 0:00 Introduction to Spontaneously Combustible Substances
• 1:40 Understanding Class 4.2 Classifications
• 3:40 Safe Storage Requirements
• 5:30 Proper Signage and Ventilation
• 6:30 Training and Risk Assessment
• 7:27 Episode Closing and Call to Action

Transcript

Speaker 1: 0:06

Hello and welcome to Chemical Safety at Work, the podcast that helps you better understand dangerous goods and the steps you can take to keep your workplace safe and compliant. I'm your host, patrick G, and today we're talking about a particularly dangerous and often misunderstood group of chemicals spontaneously combustible substances. Now, if you're working with materials like white phosphorus or aluminium alkyls, this episode is especially important. These substances can actually ignite on their own just from being exposed to air. Let's dive into what makes these chemicals so dangerous, how they're classified and how to safely store and handle them in your workplace. So what exactly is a spontaneously combustible substance? As the name suggests, these chemicals can self-ignite when they come into contact with oxygen, sometimes in just a matter of minutes. These substances fall under class 4.2 of the Australian Dangerous Goods Code, that's the division specifically for substances liable to spontaneous combustion. Class 4 is split into three divisions Division 4.1, flammable solids, division 4.2, spontaneously combustible substances, and Division 4.3, dangerous when wet. Today we've focused on Division 4.2, and there are two key categories here Pyrophoric substances, which ignite within 5 minutes of air exposure, and self-heating substances, which catch fire slowly over hours or days, especially in large quantities. These substances can be incredibly dangerous if not handled correctly. We're talking about human harm from exposure, fire and explosion risks, toxic gases released during a fire, contamination of air, land and water, reactions with incompatible chemicals and, of course, major property damage. And because of these risks, class 4.2 substances are assigned a packing group which tells you how dangerous the chemical is. There are three levels Packing group 1, high danger. Packing group 2, medium danger and packing group three, low danger. For example, pyrophoric solids and liquids are packing group one. Meanwhile, self-heating organic substances might fall into packing group two or three, depending on the level of risk. You'll find the packing group in the safety data sheet, so always check the STDS before working with any new substance.

Speaker 1: 2:46

Storage is absolutely critical when it comes to spontaneously combustible substances. The Australian standard ANNZS 5026 outlines what's required when storing class 4 dangerous goods. Let's run through the main options goods. Let's run through the main options A freestanding, roofed chemical store, a room attached to an external wall, a specially designed area inside a building with a fire resistant wall or, where appropriate, an open air storage facility and, of course, a compliant, fire resistant chemical storage cabinet.

Speaker 1: 3:27

If you're using a cabinet and many workplaces do it needs to meet very specific requirements. It must be made from double walled steel. The internal space within the walls needs an air gap of at least 40 millimeters. The bottom must contain any leaks, at least 150mm deep and liquid. Tight Shelves should be perforated to allow air movement. Doors must be self-closing and held shut by a friction or magnetic mechanism that'll release under pressure and, importantly, components must not melt under 850 degrees Celsius. All of this is designed to give you time to react if something goes wrong and to prevent a small issue from becoming a disaster. Storage cabinets must also include clear signage. Here's what you'll need A class 4.2, spontaneously combustible label at leastimetres long, no smoking, no ignition sources within three millimetres.

Speaker 1: 4:29

In letters, at least 50 millimetres high, the cabinet's maximum capacity and the name and address of the manufacturer. Now let's talk ventilation. If there's any chance of your stored substances releasing toxic or flammable vapours, mechanical ventilation is a must. That means ducting designed to prevent back pressure, and it must direct fumes away from ignition sources and anywhere where people might gather. Speaking of ignition, this one's simple but essential there must be no ignition sources within three meters of your class 4.2 cabinet. That includes sparks from equipment, hot surfaces, static discharge, even someone lighting a cigarette on their break. Make sure your team understands this and make sure they've been trained to recognize these risks.

Speaker 1: 5:23

All of this storage signage exclusions. It only works if your people understand what they're dealing with. All staff and contractors must be trained in how to safely store and handle class 4 substances and how to identify and eliminate hazards. Don't forget to carry out a risk assessment of your site. That's your roadmap for making sure every hazard is accounted for and controlled. So to wrap it up, spontaneously, combustible substances class 4.2, are extremely dangerous, but with the right systems in place, they can be stored and handled safely. Always check your SDS, choose the correct storage solution, mark your cabinets clearly, train your team and keep ignition sources well away.

Speaker 1: 6:13

Thanks for tuning into Chemical Safety at Work. If you found this week's episode helpful, please subscribe to Chemical Safety at Work on your favorite podcast platform and rate or leave us a review. We'd love to hear from all of our listeners, so send us a message and let us know if there are any topics you'd like us to do a deep dive on. Until next time, stay safe, stay compliant and always respect the risk. Until next time, stay safe, stay compliant and always respect the risk. Until next time, stay safe, stay compliant and always respect the risk. Until next time, stay safe, stay compliant and always respect the risk.