Most fires need an obvious ignition source: a flame, a spark or an electrical fault. Spontaneous combustion is different. Certain materials can generate their own heat through biological and chemical processes until they reach ignition temperature, sometimes days or weeks after they were stored. For rural fire departments and investigators in agricultural areas, hay fires are the classic example. The chapter's older training page hosted an extension article on the fire risk of wet hay; this updated overview explains how self-heating happens, the warning signs, safe response practices and what investigators should consider when spontaneous combustion is a possible cause.
How Hay Heats Itself
Freshly baled hay is still biologically active. Plant respiration and the growth of microorganisms produce heat, and a small amount of heating is normal as hay cures. The problem begins when hay is baled or stored with too much moisture. University extension services commonly advise that hay stored above roughly 20 percent moisture can generate more heat than it can safely dissipate, with the risk increasing in large, tightly packed bales and stacks.
As microbial activity raises the temperature, heat-tolerant organisms take over and chemical oxidation reactions begin. Once those reactions become self-sustaining, the temperature can continue to climb even after microbial activity stops, eventually reaching the point where the hay ignites. The process can take anywhere from a few days to several weeks after baling, which is why hay fires sometimes occur long after the hay was put up.
Warning Signs of Heating Hay
- A sharp, caramel, burnt or tobacco-like odor around the stack.
- Steam or vapor rising from bales, or condensation on the roof and walls of the barn.
- Visible mold growth on bale surfaces.
- Bales that feel hot to the touch, or a stack that is settling unevenly.
Because these signs can appear late, regular temperature checks are the best early warning.
Checking Hay Temperature
A long temperature probe driven into the center of bales or into the stack gives the best reading. The older article on this site described a simple alternative: pushing a metal rod or crowbar deep into the stack, leaving it for a couple of hours and judging the temperature by how long it can be held. A probe is far more reliable, and readings should be taken at multiple locations and repeated daily while hay is at risk.
Approximate temperature guide
Extension services publish similar thresholds. They are approximate and should be used only as general guidance:
- Below about 125°F: generally within the normal curing range, but continue monitoring new hay.
- Around 130–140°F: heating is significant; monitor at least daily.
- Around 150°F: action is needed. Many extension guides treat this as the point to take action, such as improving air circulation, while continuing close monitoring.
- Around 175°F and above: hot spots or fire pockets are likely. Call the fire department before moving hay.
- Around 185–212°F: fire is likely present within the stack, and exposure to air can cause rapid flaming.
Safe Response to a Heating Stack
Hot hay stacks are dangerous for farmers and firefighters alike:
- Do not walk on top of a heating stack. Burned-out cavities can collapse, dropping a person into a smoldering pocket.
- Call the fire department before moving very hot hay. Pulling bales apart introduces fresh air, which can turn smoldering hay into open flame.
- Have water and suppression resources ready before removing bales, and move animals and equipment away from the structure.
- Spread removed bales out in a safe area away from buildings so they can cool, and keep monitoring them.
Firefighters should also be aware that burning hay produces heavy smoke and toxic gases. Our fire scene safety guide discusses respiratory hazards.
Other Materials That Can Self-Heat
Hay is not the only material capable of spontaneous heating. Investigators regularly encounter:
- Rags and applicators contaminated with drying oils, such as linseed oil and some oil-based stains and finishes, which oxidize and generate heat when wadded or piled.
- Compost, mulch and wood chip piles, especially large, moist piles.
- Laundry contaminated with oils, particularly when warm items are piled after drying.
- Certain agricultural and industrial materials stored in bulk.
Not every oil self-heats. Petroleum-based oils generally do not behave like drying oils, so identifying the specific material involved is important.
Investigating a Suspected Spontaneous Combustion Fire
Spontaneous heating is a legitimate cause, but it must be proven, not assumed. As with any fire, apply the scientific method described in NFPA 921.
Origin
Self-heating fires usually originate within the mass of material rather than on its surface. Charring that is deepest in the interior of a stack or pile and burns outward is consistent with self-heating, but must be distinguished from fire that entered the stack from outside.
Material and history
Establish what the material was, how and when it was produced or baled, its moisture content if known, weather during harvest and storage, how it was stacked and how long it had been stored. Records such as harvest dates, bale moisture readings, purchase receipts and photographs taken by the owner can be valuable, so ask for them early. Interviews with owners and workers are essential; see interviews and canvassing.
Eliminating and testing alternatives
Evaluate other potential ignition sources in the area of origin, such as electrical equipment, lighting, vehicles and machinery, smoking, lightning and incendiary activity. Remember that eliminating other causes is not enough by itself; there must be positive evidence supporting the self-heating hypothesis.
Samples and documentation
Photograph the stack or pile and its remains systematically, collect samples of unburned material when useful for moisture or other analysis, and document storage conditions. Our evidence collection guide covers packaging and chain of custody.
Prevention Messages for the Community
Investigators often become educators after the fact. Simple prevention messages include baling hay at recommended moisture levels, monitoring new hay for several weeks, storing hay away from other buildings where possible, and disposing of oily rags safely in sealed metal containers or by spreading them out to dry according to product instructions.
Learn More
Understanding unusual ignition mechanisms is part of every investigator's professional development. Explore more investigator resources, check the Training page for upcoming sessions, and consider joining the Utah Chapter IAAI.