HF Gas: The Hidden Danger in Battery Fires
Every lithium-ion battery fire releases hydrogen fluoride. It is invisible, it acts on the body hours after exposure, and a single large pack can produce it in kilogram quantities. This guide explains where HF comes from, how much to expect, what it does to people, and which controls actually reduce exposure.
1. Why HF Is the Hazard People Underestimate
When a lithium battery fails, attention goes to what can be seen: flame jets, smoke, a glowing casing. Risk assessments follow the same instinct. They address fire spread, heat and explosion pressure in detail, then list "toxic gases" in a single line and move on. Of all the gases in that line, hydrogen fluoride (HF) is the one that most deserves a section of its own.
HF is underestimated for three reasons. First, it is invisible and, at harmful concentrations, not reliably detectable by smell before damage is done. Second, its effects are frequently delayed: a person can leave an incident scene feeling only mild irritation and develop serious systemic poisoning hours later. Third, the quantities involved are not intuitive. People picture a trace contaminant; a large battery pack in full thermal runaway can release HF measured in kilograms.
For anyone who transports, stores or recycles lithium batteries, particularly damaged, defective or end-of-life packs, this changes the question. The relevant question is not only "will the fire spread?" but also "where will the HF go, and who will be standing in it?"
Key principle: HF release is intrinsic to lithium-ion thermal runaway. It is not a sign of a poor-quality cell or an unusual chemistry. Any credible plan for handling lithium batteries at scale must treat HF as a certainty in the event of failure, and must control it at the source rather than relying on dilution after release.
2. The Chemistry: From LiPF₆ to HF
Thermal runaway is a self-sustaining sequence of decomposition reactions that takes a cell from around 80 °C to several hundred degrees within minutes, ending in rupture and venting. The full cascade, and the overall composition of the vent gas, is covered in our engineering guide Thermal Runaway: What It Is and How to Contain It. Here we focus on one strand of that chemistry: how fluorine in the electrolyte becomes hydrogen fluoride in the air.
The electrolyte in virtually every commercial lithium-ion cell consists of organic carbonate solvents carrying a dissolved conducting salt. The dominant salt is lithium hexafluorophosphate (LiPF₆). It is chosen for good reasons: high ionic conductivity and the ability to passivate the aluminium current collector. It is also thermally fragile and highly sensitive to water.
As the cell heats, three routes lead to HF:
- Thermal decomposition of the salt. LiPF₆ breaks down into lithium fluoride (LiF) and phosphorus pentafluoride (PF₅). PF₅ is a strong Lewis acid and extremely reactive.
- Hydrolysis. PF₅ and residual LiPF₆ react with any available moisture, whether trace water inside the cell, water produced by solvent decomposition, or humidity in the surrounding air once the cell vents. These reactions produce HF directly.
- The POF₃ intermediate. PF₅ reacting with water or with oxygen-containing solvent fragments yields phosphoryl fluoride (POF₃). POF₃ is itself highly toxic and in turn hydrolyses to further HF. Measured HF downstream of a fire therefore includes HF formed from POF₃ well after the gas has left the cell.
Other fluorinated components contribute as well. The PVDF binder that holds electrode coatings together contains fluorine and releases HF when it decomposes at high temperature, and some electrolyte additives are fluorinated. The practical result is the same: fluorine is built into the cell, and thermal runaway liberates it.
Moisture makes it worse, not better. Because hydrolysis is a primary HF formation route, humid air, condensation and applied water all feed the reaction. This is counterintuitive for anyone trained on conventional fires, where water is the default response, and it has direct consequences for firefighting tactics and post-event clean-up, discussed in sections 6 and 8.
Chemistry matters for the quantity of HF, but not for whether it appears. LFP cells, which run cooler and vent less violently than NMC or NCA, still use LiPF₆ electrolyte and still release HF. Changing cathode chemistry changes the fire behaviour; it does not remove the fluoride hazard.
3. How Much HF? The Larsson Data and Real Pack Sizes
The most widely cited quantitative work on HF from lithium-ion fires is by Larsson et al., published in Scientific Reports in 2017. The researchers subjected a range of commercial cells and battery packs to controlled fire exposure and measured the fluoride species in the resulting gases. Their key finding was that HF release scales with the energy content of the battery, and falls within a band of roughly 20 to 200 milligrams of HF per watt-hour of nominal cell capacity. They also detected POF₃ in some tests, confirming that the intermediate is released in practice and not only in theory.
The tenfold spread reflects real variation between tests: cell chemistry, state of charge, cell format, and whether the vent gases ignited and burned or were released unburned all influence the yield. For planning purposes the range is what matters, not a single point value, because the conditions of an actual incident are never known in advance.
3.1 Scaling to Real Pack Sizes
Laboratory tests use cells and small packs. The batteries moving through logistics chains today are far larger. Multiplying the Larsson range by pack capacity gives a sense of scale. For a 90 kWh pack:
90,000 Wh × 20 mg/Wh = 1.8 kg | 90,000 Wh × 200 mg/Wh = 18 kg
The same arithmetic across typical battery sizes gives the following. These figures are order-of-magnitude illustrations only, obtained by linear scaling of small-scale test results. They are not measurements of the listed pack types, and actual release in a given event may fall outside the range.
| Battery type (illustrative) | Energy content | Estimated HF at 20 mg/Wh | Estimated HF at 200 mg/Wh | Scale of concern |
|---|---|---|---|---|
| Smartphone | ~15 Wh | ~0.3 g | ~3 g | Local |
| E-bike battery | ~0.5 kWh | ~10 g | ~100 g | Room-scale |
| Hybrid or small EV module | ~10 kWh | ~0.2 kg | ~2 kg | Serious |
| Mid-size EV pack | ~50 kWh | ~1 kg | ~10 kg | Severe |
| Large EV pack | ~90–100 kWh | ~1.8–2 kg | ~18–20 kg | Severe |
| Commercial vehicle or BESS unit | ~200 kWh | ~4 kg | ~40 kg | Critical |
3.2 What a Kilogram Means
Masses are hard to relate to exposure limits, so it helps to convert. HF has a molar mass of about 20 g/mol, so one gram is roughly 0.05 mol, which occupies about 1.2 litres as a gas at room temperature. Diluting 1.2 litres of HF to the 30 ppm IDLH threshold requires around 40 m³ of air. One kilogram, evenly mixed, would bring roughly 40,000 m³ of air to IDLH concentration.
This is an idealised calculation. In a real event HF does not mix evenly: some of it reacts with surfaces, deposits on soot and debris, or dissolves in water, which lowers the airborne total. At the same time, the plume close to the source is far more concentrated than the average, and HF formed from POF₃ appears downstream. The conclusion is robust regardless: a single failed EV pack carries enough fluoride to make a very large volume of air immediately dangerous, many times larger than a trailer, a container or a ship's hold.
4. What HF Does to the Human Body
Hydrogen fluoride is unusual among industrial toxicants because it causes harm in two distinct ways. It is a corrosive, damaging tissue at the point of contact, and it is a systemic poison, because the fluoride ion penetrates deep into tissue and into the bloodstream before it causes damage.
HF is extremely soluble in water. It dissolves readily in the moisture of the eyes, the airways and the skin. Unlike many acids, it is only weakly dissociated, which allows the undissociated molecule to pass through skin and mucous membranes and into deeper tissue. There the fluoride ion binds calcium and magnesium, forming insoluble salts, killing cells and depleting the body's circulating calcium.
The delay is the danger. The IDLH (Immediately Dangerous to Life or Health) value for hydrogen fluoride published by NIOSH is 30 ppm, and occupational exposure limits in most jurisdictions sit in the low single-digit ppm range. Yet symptoms are frequently delayed by hours. A driver, warehouse operative or first responder may feel only a sore throat or itching skin at the scene, decline assessment, and develop pulmonary oedema or cardiac complications later that day. Anyone with suspected HF exposure requires medical assessment, regardless of how they feel.
Two practical consequences follow. First, absence of symptoms at the scene is not evidence of absence of exposure, so incident procedures must record who was present and route them to medical assessment. Second, the treating clinician needs to know that HF was involved. "Battery fire, smoke inhalation" does not prompt the same treatment as "lithium battery thermal runaway, suspected hydrogen fluoride exposure". Treatment for HF focuses on neutralising fluoride and restoring calcium, and depends on the clinician knowing to look for it.
5. Enclosed Spaces: Trailers, Holds, Warehouses and Why Dilution Fails
In open air, a venting pack's HF plume disperses, and concentrations fall with distance. That is the scenario most people imagine. It is not where most batteries spend their time. Batteries in the supply chain sit in enclosed volumes: curtain-sider and box trailers, shipping containers, ships' holds, rail wagons, warehouse bays and collection points.
Compare the calculation in section 3.2 with typical enclosed volumes. A 40-foot shipping container has an internal volume in the region of 70 m³. A standard semi-trailer box is of a similar order. Even at the low end of the Larsson range, a large EV pack carries enough fluoride to take the air in such a space far beyond IDLH many times over. The limiting factor is not how much HF is generated, but how fast it escapes, and in an enclosed volume that is slow.
5.1 Why Dilution Is Not a Control
General ventilation is designed for routine air quality, not for a point source releasing kilograms of acid gas over minutes to hours. Several factors defeat it:
- Rate mismatch. Peak HF release coincides with the most violent venting phase. Building ventilation rates, expressed in air changes per hour, cannot track a release profile measured in minutes.
- Stratification and pockets. Hot fire gases rise and accumulate under roofs and in the upper part of holds; as they cool, contaminated air settles unevenly. Average concentration is a poor guide to what a person entering the space will breathe.
- Surface loading. HF deposits on walls, racking, cargo and soot. The air may clear while surfaces remain contaminated and continue to off-gas, particularly as humidity changes.
- Displacement, not removal. Extraction moves HF somewhere else. In a warehouse that may be the yard or the neighbouring unit; on a ship it may be the ventilation path to accommodation.
Maritime carriage deserves particular attention. A ship's hold is large, closed, remote from shore-side fire services, and connected to spaces where crew live and work. Battery fires at sea cannot be abandoned or left to burn out at a safe distance. For a comparison of how sea and air regimes approach these risks, see IMDG vs. IATA: Key Differences for Battery Shippers.
Damaged batteries concentrate the risk. The packs most likely to fail in transit and storage are the ones already damaged, defective or at end of life. These are routinely consolidated in collection points and moved in full loads to recyclers, which puts the highest-risk batteries into the most enclosed spaces at the highest density. Correct classification is the first step; our guide Who Wants to Be Yellow? Classifying Damaged Lithium Batteries under Special Provision 376 explains how.
Volumes of these batteries are rising. Extended producer responsibility and recycling targets under the EU framework mean more end-of-life packs moving through logistics networks each year, as outlined in EU Battery Regulation 2026: Impact on Logistics. The enclosed-space HF problem will grow with them unless the hazard is addressed at the package level.
6. Mitigation Options Compared
Several measures are commonly proposed for managing battery fire gases. They differ fundamentally in whether they reduce the amount of HF that reaches people, or only change where it goes or when someone finds out about it.
| Measure | What it does | Reduces HF reaching people? | Main limitation |
|---|---|---|---|
| Ventilation / extraction | Dilutes and moves contaminated air | Partial, dilution only | Cannot match peak release rate; displaces HF elsewhere; unavailable in most transport settings |
| HF gas detection | Electrochemical sensors raise an alarm at set concentrations | No, warns only | Useful for evacuation and re-entry decisions, but does not remove a single molecule of HF |
| Water spray / deluge | Cools surroundings; can scrub some HF from the air | Partial | Produces HF-contaminated runoff; moisture feeds hydrolysis; not viable in transit |
| Filler materials (sand, vermiculite) |
Thermal mass and physical separation | No | No chemical effect on HF; contaminated filler becomes hazardous waste |
| Respiratory PPE | Protects the individual wearer | For wearers only | Relevant for trained responders; no protection for bystanders, drivers or neighbours |
| Chemical filtration at source (sealed enclosure + neutralising layer) |
Neutralises HF in the vent stream before gas leaves the package | Yes | Must be built into certified containment packaging; filtration layer is consumed and replaced after an event |
The hierarchy of controls applies here as in any occupational hazard. Detection and PPE sit at the bottom: they are necessary, but they manage people's exposure to a hazard that has already been released. Ventilation and water are engineering controls, but act on the hazard after it has mixed with the air people breathe. Only control at source, capturing and neutralising HF before it leaves the package, reduces the quantity available to cause harm.
Detection is a complement, not a solution. Electrochemical HF sensors are a sound investment in warehouses and charging areas, and they give responders objective data for re-entry decisions. But an alarm tells you that HF is already in the air. Detection should sit alongside source control, not replace it.
7. How BUNCKER® Handles HF
BUNCKER® containment is designed on the premise that a battery inside it may go into full thermal runaway, and that the gas it produces, not only its heat, has to be managed. Three design elements work together.
- Sealed bell jar architecture. The patented enclosure lowers over the battery and seals to its base. Vent gas cannot escape through gaps, lids or seams; it follows a defined path out of the enclosure. This is what makes treatment of the gas possible: you can only filter a gas stream that you control.
- Fibercon Altilium® CP Blanket (FACB). The vent path runs through the FACB layer, which chemically neutralises hydrogen fluoride in the vent stream before the gas exits the enclosure. The gas that leaves is depleted of its most dangerous constituent, rather than simply diluted into the surrounding air.
- Thermal decoupling. The multi-layer wall construction keeps the outer surface below 100 °C throughout a full thermal runaway event. This matters for HF as well as for fire: a unit that does not ignite its surroundings does not add secondary fires, and secondary fluoride sources, to the incident.
Because the enclosure manages heat and gas structurally and chemically, no sand, vermiculite or other filler is required. That keeps the fluoride problem contained to the unit and the FACB layer, rather than spread through tonnes of contaminated filler that must later be handled and disposed of.
All six models in the Pro Line Heavy range carry UN LP906 certification for lithium batteries of every type and condition, including damaged, defective and end-of-life, and are valid for road (ADR), sea (IMDG) and rail (RID) transport as well as indoor and outdoor storage. The range runs from the SDR (45.8 kWh, side loading system) through the S (50 kWh), M (90 kWh), L (90 kWh) and XL (100 kWh) to the XXL (200 kWh), the last five using a pallet loading system. For what LP906 certification requires in testing, see Understanding UN P911 & LP906 Certification.
Section 3 showed that HF scales with energy content. The same logic applies to specifying containment: size the unit by the full kWh capacity of the pack, and assume that the full fluoride inventory of that pack may be released. A 90 kWh pack belongs in a unit rated for at least 90 kWh, with filtration designed for that event.
BUNCKER® Pro Line XL: 100 kWh Containment
Patented bell jar architecture with integrated Fibercon Altilium® CP Blanket HF filtration, neutralising hydrogen fluoride in the vent stream before gas exits. UN LP906 certified for all lithium battery types, with no filler material required and an outer surface below 100 °C throughout a full thermal runaway event. Made in the Netherlands by VDL.
View product →
8. Operational Guidance: Response, PPE, First Aid and Residues
Source control reduces the hazard; it does not remove the need for sound procedures. The guidance below is general and must be integrated with your own emergency plan, your local fire service's requirements and qualified medical advice.
Keep calcium gluconate gel where batteries are handled. Sites that store, repair, dismantle or consolidate lithium batteries should hold calcium gluconate gel in their first aid provision, train first aiders in its use for HF skin exposure, and brief local medical services that HF exposure is a foreseeable risk at the site. The gel is a first aid measure; it does not replace medical assessment.
On water: firefighters may still use water to cool surroundings and protect exposures, which is a legitimate tactic applied by trained responders. What should be avoided is untrained application of water to an unsealed, venting pack in an enclosed space, which generates contaminated runoff and adds moisture to the hydrolysis reactions. Any runoff should be contained where possible and treated as contaminated.
After an event inside a BUNCKER® unit, contact the manufacturer's service programme for inspection, decontamination and recertification. The FACB layer is consumed in neutralising HF and must be replaced before the unit returns to service. For how containment fits into a complete return and recycling chain, see EV Battery Return Logistics: A Case Study.
Frequently Asked Questions
Frequently Asked Questions
Effectively, yes. Essentially all commercial lithium-ion cells use a fluorinated conducting salt, most commonly LiPF₆, and most also contain a fluorinated binder. When the cell goes into thermal runaway these compounds decompose and react with moisture to form HF. The amount varies with chemistry, state of charge and fire conditions, but HF release is intrinsic to the technology, not a sign of a defective or low-quality cell.
Fire tests by Larsson et al. (Scientific Reports, 2017) measured roughly 20 to 200 mg of HF per watt-hour of cell capacity. Scaling that range linearly to a 90 kWh pack gives an order-of-magnitude estimate of about 1.8 to 18 kg of HF. This is an illustration rather than a measurement of any specific vehicle, but it shows that a single large pack can release HF on the order of kilograms.
HF penetrates skin and lung tissue before it causes damage, and much of its harm comes from fluoride binding calcium and magnesium in deeper tissue and in the blood. Pulmonary oedema and cardiac effects can develop hours after exposure, while initial symptoms may be limited to mild irritation. This is why anyone with suspected HF exposure needs medical assessment even if they feel well.
No. Electrochemical HF sensors are valuable for raising the alarm and for deciding when a space is safe to re-enter, but detection does not reduce the amount of HF released. By the time a detector alarms, HF is already in the air. Detection should be combined with controls that limit release at source, such as certified containment with integrated gas filtration.
That is a decision for trained fire responders. Water is used to cool surrounding structures and neighbouring cargo, but applied to a venting pack it produces HF-contaminated runoff and adds moisture to the reactions that form HF. Untrained personnel should not apply water to a venting battery in an enclosed space. Any runoff should be contained where possible and treated as contaminated.
In a BUNCKER® unit, the sealed bell jar forces vent gas along a defined path through the FACB layer. The blanket chemically neutralises hydrogen fluoride in the vent stream before the gas exits the enclosure, so the gas that leaves is depleted of HF rather than simply diluted. The layer is consumed in the process and is replaced during post-event service and recertification.
Remove the person from the contaminated area, remove contaminated clothing, and flush the skin thoroughly with water. Calcium gluconate gel is then applied to the affected area to bind fluoride. Eyes should be irrigated continuously with water. In every case the person must receive medical assessment, and the clinician must be told that hydrogen fluoride is the suspected agent. Follow your site's first aid procedures and qualified medical advice.
Stop HF at the Source
BUNCKER® units are UN LP906 certified for every lithium battery type and condition, with a sealed bell jar and Fibercon Altilium® CP Blanket that neutralises hydrogen fluoride before gas leaves the enclosure. Our specialists can size the right model for your packs and your operation.
2 Comments
Comments are closed.