Button batteries are treated as an emergency out of proportion to their size, and the reason is the mechanism of injury. The harm is electrical and chemical, not mechanical.
The injury is caused by current, not leakage
When a battery lodges against moist tissue, the tissue completes a circuit between the two terminals. Current flows, and that current splits water into hydrogen and hydroxide ions.
Hydroxide accumulating at the negative terminal makes the local tissue strongly alkaline. Alkaline injury dissolves tissue rather than sealing it, so damage continues to spread inward.
The common belief that the danger is acid leaking from a corroded cell is mistaken. A battery that still holds charge is more dangerous than a depleted one, not less.
Time is the critical variable
Because the process starts as soon as contact is made, injury can begin well before any symptom appears. The relevant clock starts at ingestion, not at the point a parent notices.
This is why emergency guidance is unusually direct about not waiting to see whether a child seems unwell. The interval between swallowing and serious injury can be short.
Larger cells lodge more easily in a small child's oesophagus, which is where the most serious injuries occur. Batteries that pass into the stomach behave differently and are assessed separately.
The early signs resemble ordinary illness
A child may cough, drool, refuse food, complain of chest or throat discomfort, or simply appear unsettled. Every one of those is far more commonly caused by a virus.
There is often no witnessed swallowing event, so the parent has no reason to connect the symptoms to a battery. This is the single largest cause of delay.
Because the presentation is non-specific, the practical safeguard is knowing which devices in the home contain these cells rather than relying on recognising symptoms.
The sources in a home are not obvious
Button cells sit in remote controls, car key fobs, kitchen scales, thermometers, hearing aids, flameless candles, musical greeting cards and a large number of small illuminated novelties.
Regulated toys generally use screwed compartments, but many household items do not, and greeting cards and novelties are frequently the least secured items in the house.
Loose spare batteries and recently removed dead ones are a distinct hazard, since they are handled by adults and set down within reach rather than stored.
What the response looks like
Swallowing or suspected swallowing of a button battery is treated as an immediate emergency by paediatric and poisons services, meaning emergency care rather than a routine appointment.
Guidance on what to do while travelling to hospital varies by country and has changed over time, so it should come from your local emergency service or poisons centre rather than from memory.
Prevention is straightforward by comparison: know where the cells are, keep spares and dead cells out of reach, and check that compartments in frequently handled devices are secured.