It is a common misconception that the air inside caves is bad: bad smells and low oxygen. Generally, the opposite is true. Let me explain.
What does a cave smell like? Caves smell somewhat musty and earthy. The air is mostly damp, with 80 - 90% humidity. Some caves will smell a bit moldy. Caves with a stream running through will smell very fresh, with clean, clear air. Some caves have bad air, although this is uncommon. Most caves have healthy, clean air.
Most caves don't smell the way people expect them to. They smell fresher than you might think. Also, cave air is actually very healthy. But there are some situations to be aware of.
Different Caves Smell Differently
This probably won't be a surprise to you. Different caves smell differently. But they almost always have some basic scent to them, which cuts through any other odors that linger on top. So what does this 'cave perfume' smell like exactly?
Let me try to describe it:
Basic smell: The basic cave odor is muddy, musty and earthy. The air is damp and cold. But at the same time, cave air is surprisingly light on the lungs.
Wet cave: If the cave has a stream running through it, the air is fresh, clear, cool and odorless. Wet caves without running water are more likely to smell moldy.
Dry caves: Dry caves don't smell like a whole lot, because there's nothing there to stink up the place. When the humidity of the cave drops, the air gets less heavy and doesn't smell as much. Also, without water, the smell of mud gets less pronounced.
Bats: If the cave has bats, you will smell them. But bats mostly live just around the cave entrance. Bats themselves don't stink - according to bat researcher Amanda Lollar, they actually smell weirdly pleasant. What doesn't smell pleasant, is the accumulation of guano and urine on the ground below. Any caver that has been inside a bat cave knows this scent - it's like that gigantic rabbit cage you haven't cleaned for a couple of weeks. (By the way, you shouldn't enter bat caves.)
Climate: The climate of the region also affects smell a lot. Wetter climates (or even below sea level - like here in The Netherlands) will smell a lot more musky and muddy. In comparison, mountain caves will smell fresher, cleaner, clearer. Also, temperature plays a role.
Cave Air is Actually Very Healthy
Most people think that cave air is bad for you. This is another misconceptions about cave air: the opposite is true. Just 1% of caves has bad air. So 99% has good air, or even great air.
Most cave air is actually very healthy. In Austria, caves are being used to cure respiratory illnesses.
The reason is the higher humidity inside the cave. It helps to settle the tiny, atmospheric dust particles. There are usually also no allergens.
This is why people are often surprised when they enter a cave for the first time. The air is actually weirdly clean and clear.
Where Does the Smell Come From?
The smell of caves comes mostly from the rocks, in combination with the humidity. Most caves are pretty damp. It's pretty common that a cave has an 80 - 90% humidity rate.
The reason it doesn't smell moldy is mostly that there doesn't live a lot inside the cave. There aren't any plants to rot or animals with strong odors (expect for bats).
So why don't caves stink as bad as most people think? Well, the air flow in natural caves is usually pretty good. There's plenty of fresh air coming in. And especially if there's some sort of stream, the air can be pretty amazing.
There are a couple of reasons the air flow of caves is pretty good:
- caves have a lot of entrances (small and large)
- difference in pressure and temperature create an air current
- whenever the difference gets bigger, the current increases
- so the cave tries to manage air disparities between different sections
- in a way, the cave has its own climate control system
So whenever there's bad air, it gets fixed by the cave. This means that with most cave systems, the air is of pretty good quality throughout the system - and is very stable.
Mines Are the Complete Opposite
Man-made caves smell a lot worse than natural caves. Remember, air flow in natural caves is pretty good. In mines, not so much. Mines contain a lot of artificial pockets where there isn't any proper air flow. Mine tunnels sometimes don't have multiple entrances. This can result in low oxygen, which is dangerous.
Some rocks release gases when they're mined. A great example is the carbon mines, where carbon gases would build up until there was a 'fire damp gas', as it's called. This damp is lightly flammable and highly explosive.
Other Cave Systems With Bad Air
Some cave systems do have bad air. Some cave types you might want to avoid:
Closed chambers - Closed off chambers can contain CO2 pockets that build up over time. Especially in very warm (tropical) climates.
Sunken pockets or chambers - Small sunken pockets in a passage might contain low oxygen. CO2 is heavier than air, which means it will build up in the lowest pockets in a cave system. However, if the cave has a decent air flow, this won't be a real problem. If you plan on resting for a bit, it's better to get up to the high ground.
Bat caves - Chambers that are used by bats can contain ammonia. The decomposing guano creates ammonia gases.
Flood caves - Entering flood caves takes a lot of precaution as is, but the air can also be bad. A good rule of thumb is that any cave that gets closed off periodically (due to flooding), might not have the best air flow. So CO2 can build up over time.
Tropical caves - Very dry, tropical caves can grow the fungus Histoplasma. This fungus, also called caver's disease, grows on bat guano. It can cause serious breathing problems.
Volcanic caves - Volcanic caves can contain sulfur, which is very damaging to your lungs and skin.
Smells (and Signs) To Be Aware Of
Some caves have 'bad air' - which simply means the air is toxic, and you should avoid breathing in the air.
Signs to look for (and smells to be aware of) are:
- fatigue - could indicate low oxygen
- strong, pungent smell - indicates high levels of ammonia
- rotten eggs - indicates high levels of sulfur
Low oxygen is difficult to spot: people think a candle or carbide lamp will go out before the levels of CO2 get life-threatening. This is actually not true. You will lose consciousness before the carbide lamp goes out.
So be very aware of your energy levels - if you are feeling unnaturally fatigued, simply return to the entrance.
Why are caves cool? Caves are cool because of the evaporation process. Caves inhale cold, dry air from outside. This drier air evaporates water from the damp air inside the cave. This cools the cave temperature further, because evaporation requires a lot of heat.
Why is oxygen scarce in caves? A common misconception is that caves contain low oxygen. The vast majority of caves have very healthy air with plenty of oxygen. The air flow in most caves is very good, resulting in very stable and predictable oxygen levels.