Heat and humidity indoors: which thresholds actually matter

Companion animals lose heat in four ways: radiation, conduction towards cooler surfaces, convection with moving air, and evaporation. Dogs rely mostly on evaporation through panting, that is the rapid passage of air over the moist surfaces of the mouth and airways. Birds use an analogous mechanism. Cats depend far more on conduction, seeking cool surfaces, and on reducing activity. Rabbits and rodents have limited dissipation capacity and no efficient equivalent of sustained panting.

Relative humidity determines how much more water vapour the air can take up. When it is high, evaporation from mucous membranes slows and panting becomes progressively less effective: the animal keeps spending energy to breathe without gaining cooling, and the metabolic heat produced by that respiratory effort worsens the balance. This is why a day at twenty eight degrees with very humid air is more dangerous than one at thirty two with dry air. A thermometer on its own therefore does not describe the risk: it has to be read alongside a hygrometer.

The value that counts is not the one on the hallway thermostat, but the air the animal breathes where it spends the day. Between a sensor at head height in a ventilated room and the floor of an enclosed conservatory there can be several degrees of difference, and the difference runs the wrong way for anyone living twenty centimetres above the ground or inside a cage in the sun.

• Place a thermometer with hygrometer at the animal's height, not at human height, and where it actually sleeps, which is often not the spot you chose.
• Measure the worst place in the house too, typically the room facing the afternoon sun, a conservatory, a windowless bathroom or the room where the animal is shut in when you go out.
• Record the daily maximum, not the reading at the moment you come home: most inexpensive devices store minimum and maximum, and the maximum is what you need.
• Check surface temperatures, not just air: a windowsill, a floor under a window or the metal base of a cage can be far hotter than the surrounding air.
• For bird and small mammal cages, measure inside the cage rather than in the room, because the position relative to the window changes everything.
• Log the values for a few days before intervening: knowing when the house peaks tells you when to act, and acting early costs less than fixing it afterwards.

The values in the table are general reference ranges for healthy adult animals in air of moderate relative humidity, expressed in degrees Celsius. They are not clinical limits and do not replace the assessment of a vet, who takes into account breed, body condition, current disease and the habits of the individual. They serve to tell you when the home is leaving the zone in which the animal can regulate on its own.

For relative humidity the reference band for most domestic mammals runs from 40 to 60 per cent. Above 70 per cent evaporative heat loss becomes noticeably less effective and the thresholds in the table should be treated as several degrees lower. Below 30 per cent, typical of heated homes in winter, different problems appear: dry mucous membranes, irritated skin, and worsening of chronic respiratory conditions in dogs and birds.

Many popular solutions do little, work for a few minutes only, or cause harm. The guiding principle is that the animal must be able to choose: a thermally varied home, with cool spots that are reachable and unoccupied, protects better than a home managed at a single average temperature.

• Ventilating early and closing up in time works: open at night and in the first hours of the morning, then close and shade before the sun hits the glass.
• External shading beats internal shading. Shutters, awnings and screens in front of the glass stop heat before it enters; an internal curtain stops it once it is already in.
• A fan helps humans because we sweat over the whole body surface. In dogs and cats the effect is much smaller, and a direct draught onto a bird or small mammal cage is harmful.
• Cool surfaces are the real tool for cats and many dogs: uncovered tiles, an area free of rugs, a cooling mat that is not buried under a blanket.
• Water in several places, in wide shallow bowls, refreshed more than once a day. A single bowl in the kitchen is useless if the animal is shut somewhere else.
• Lightly wetting the coat on paws, belly and armpits helps when there is air movement afterwards. Covering the animal with a wet towel instead traps heat and makes things worse.
• Do not shut the animal in small unventilated rooms for convenience: a windowless bathroom or a utility room reaches values you would not expect within a few hours.
• If you use air conditioning, aim for a moderate difference from outdoors and a flow that is not directed at the animal, while still leaving it a room at an intermediate temperature.

High humidity does more than hinder thermoregulation. It favours dust mites and moulds, worsens dermatitis in skin folds and ears, and keeps bedding fabrics damp, creating an environment that suits yeasts. In poorly ventilated flats the problem is chronic and shows up more in autumn than in summer, when windows stop being opened but indoor temperature stays mild.

Low humidity produces the opposite effect and mainly concerns the heating season. Dry nasal and ocular membranes reduce the natural defence of the airways, skin flakes, and in birds, whose respiratory system is both highly efficient and highly exposed, constantly dry air contributes to chronic respiratory problems. The correction here is not a humidifier running at full power, but a measured value held steady.

1. Measure before correcting: acting on a humidity level you have never read simply moves the problem from one extreme to the other.
2. Ventilate briefly and often rather than at length once a day, because effective air exchange takes a few minutes with two openings in communication.
3. Dry fabrics and beds fully after washing and do not leave them on cold surfaces, where they hold condensation.
4. Keep cages and carriers away from sources of moisture and from cold walls, which are where condensation forms first.
5. If you use a humidifier, clean it as instructed and do not leave it running unattended: a neglected tank becomes a source of air contamination.
6. Review where resting places sit when the season changes, because the best spot in winter is almost always the worst one in summer.

Behavioural signs come before clinical ones, and they are the most useful because they appear while there is still time to act. An animal that moves its resting place, seeks bare flooring, stretches out with limbs splayed instead of curling up, spontaneously reduces play and drinks more is telling you the environment is at the edge of its comfort zone.

The last two rows belong to the article on the first minutes of heatstroke rather than to this one. One point is worth repeating, though: almost every case of heatstroke that happens indoors arises in avoidable circumstances, namely a closed room, a conservatory, a carrier left in the sun, or a cage that was in shade in the morning and no longer was by two in the afternoon.

When a prolonged spell of heat is forecast, there are few things to do and they must be done before the house warms up, because a building that has stored heat for three days does not cool down with one night of open windows.

1. 1

   Identify the coolest room and clear it

   Usually the one facing north or on the lowest floor. Take out rugs and thick cushions, leave it accessible all day, and make sure a draught cannot swing the door shut.

2. 2

   Move cages and resting places away from glass

   The sun moves. A spot in shade at nine in the morning can be in full sun at three in the afternoon, and an animal in a cage has no way of relocating.

3. 3

   Multiply water points and check them twice a day

   Wide shallow bowls in several rooms, changed often. For cats, who often drink little, a fountain or a bowl placed away from the food increases intake.

4. 4

   Reschedule outings and activity

   Before dawn and after sunset, on shaded routes and surfaces that are not scorching. Move indoor play to the cool hours too, because it produces as much heat as a walk.

5. 5

   Weigh and record once during the spell

   Weight loss, a marked drop in food intake or a disproportionate rise in thirst are useful information for the vet, and in an animal with kidney or heart disease they should be reported straight away.

6. 6

   Prepare the plan for your absence

   If you will be out for several hours, decide in advance which rooms stay open, who checks in and when, and what happens if the power fails and the air conditioning stops.