Cage ammonia: adding bedding does nothing, a bigger cage does
Published by Planète Pets · managing editor: Clément Lacaille · how our guides are written
When a cage smells, the reflex is to add bedding. An experiment published in 2023 in Lab Animal by Eskandarani, Hau and Kalliokoski shows that reflex does not work: across a fourteen-day cycle, increasing or reducing bedding by 50% does not change measured ammonia concentrations. Our housing guides sit in the rodent bedding and habitat section.
Contents · 5 sections
What counts is air volume, not floor area
The same work delivers the most useful result for choosing a cage. At comparable animal density, concentrations stayed lower in the larger cages. In other words, the decisive parameter is not available floor area but the volume of air above the bedding, which dilutes the ammonia produced. A low wide cage and a tall one with the same footprint do not behave alike. Small cages housing more than four mice, or a breeding pair, frequently exceeded 50 ppm by the end of the cycle.
The substrate shifts the timetable
Ferrecchia, Jensen and Van Andel compared four substrates in 2014 in the Journal of the American Association for Laboratory Animal Science. The rise became significant from day 8 on alpha-cellulose, passing 100 ppm on day 11, against day 14 for corncob. The recycled paper pulp tested failed to contain ammonia and came with histological lesions within a week in static cages. The usual reference point sits below 25 ppm, close to human occupational exposure limits.
Why this is not just about smell
Gamble and Clough established as early as 1976, in Laboratory Animals, that the ammonia levels routinely reached in cages are enough to cause changes to the rat's tracheal epithelium (see the study). Ammonia degrades the airway's first line of defence, which connects directly to the conditions described in our article on respiratory mycoplasmosis in rats.
A transfer to be made carefully
All these measurements come from laboratory cages, either mechanically ventilated or closed by a filter top. A domestic barred cage is far better ventilated: the ppm figures do not transfer as they stand, and nobody has published an ammonia measurement inside a commercial hamster or guinea pig cage. The species involved are mice and rats. What does transfer is two principles: the relative ranking of substrates and the role of air volume. Nothing here sets a cleaning frequency — a point covered, conversely, in our article on cleaning the cage too often.
- ±50% bedding: no measured effect on ammonia.
- At equal density, larger cages stay less loaded: air volume dilutes.
- Day 8, then 100 ppm by day 11 on alpha-cellulose, against day 14 for corncob.
- Below 25 ppm: the usual benchmark, taken from human exposure limits.
In practice, a taller cage with a well-ventilated upper level beats a wide closed tub of the same footprint, and a solid lid on a plastic tub is the worst case. Substrate choice itself is covered in our guide to safe wood shavings and, for heavy bedding users, in the one on shavings or paper for guinea pigs.
Frequently asked questions
Should I add bedding to limit odours?
Published measurements show no effect from a 50% change in either direction. Bedding depth serves other needs, such as burrowing.
Does a big cage smell less?
For the same number of animals, yes, in the available measurements — thanks to the air volume that dilutes, not the floor area.
Do these ppm values apply to my barred cage?
No. They come from far less ventilated laboratory cages. Only the substrate ranking and the role of volume transfer.
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This guide is part of Planète Pets’s Rodents universe. Our advice is general in nature: for any health concern, your veterinarian remains the only reference.
