I have long wanted to see a lava tube because lava tubes are primary caves, and thus different from any other cave I have been in before. Our trip to Tenerife offered a chance, and it turned out that as we wanted something else than a tourist trip, Cueva de san Marcos on the northern coast of the island would be our best option. Many caves on Tenerife are gated, and Cueva del Viento, one of the biggest lava tubes in the world, is a tourist cave and thus not visitable by your own.

And to Cueva de San Marcos we went. The path to the cave was more demanding than the cave itself but nevertheless I liked our little caving trip. First we followed the cave to the other entrance, and found the geocache which is placed near that end. The other entrance is in the middle of thick vegetation, so we didn't try to go out. After that we took the left hand side passage near the entrance which leads further into the cave. After a short hands-and-knees crawl there was a pile of boulders where we had to climb up. After that we continued about 50 metres along the big passage. The air was warm and it felt very thick. Eventually we both noticed that we had started to feel unwell: we experienced a little shortness of breath, my partner got a headache, and I felt nauseous. That's where we decided to turn back, and when I was walking back towards the boulders, I noticed that my feet were shaking a little, and I really wanted to get back to better air. After we reached the boulders, it was easy to breathe again, and my nausea subsided. We also felt that it was colder under the boulders where the crawl starts.

This was the first time have experienced anything like that in a cave. I still don't know whether it was just CO2, or some gases produced by volcanic activity. 

Anyway, we liked the cave because we haven't seen any primary caves before. It was also very useful to see what it feels like when there's bad air in a cave. At first it just felt a little warm there, and we didn't notice that something was wrong but as we continued walking, we realised that it didn't feel like as it should.

Having visited Cueva san Marcos in the past (we did the through trip from the banana plantation to the sea cliffs) I am surprised to hear of bad air because there should be good air flow through the cave. The caves do tend to be humid. If it was anything it might be CO2 or just smelly rotting vegetation. Volcanic gases seems highly unlikely. It is an interesting site with some unusual melt features in it including a place where one passage has melted down into another.

I think it was maybe too much CO2 then. We didn't feel any air current after we had climbed up those boulders. The air was thick, warm and humid - different from any other parts of the cave (we didn't try the right hand side passage though). The shortness of breath and nausea weren't very good signs, and that's why we turned back.

When we've hit CO2 buildup we've also started sweating heavily - other systems as you describe? Did you?

Thick warm humid air in lava tubes is common, especially if surface conditions are sunny and there's been rainfall recently (the latter being more likely in winter months, or in the north of Tenerife pretty much any time of year!), this is because many lava tubes are close to the surface and can have high ambient temperatures, in side passages where there is less significant through flow of air, it is highly noticeable when entering a relatively "airless" passage. There are many lava tubes where this is a characteristic. High ambient temperatures combined with high humidity can make for an oppressive environment.

Experienced cavers tend to be less prone to CO2 and can become accustomed to quite high levels, apparently. See:-
http://www.cscc.org.uk/wiki/doku.php?id=information:co2

Thank you for the replies. The weather wasn't very warm - the coldest winter for years on Tenerife. It had been raining but it didn't rain that day. We have been caving a lot in limestone caves but this was our first time in a lava tube. The other parts of the cave felt normal, and the air was cool and dry. Just in that big passage it felt different, and made us feel unwell. As I said, at first I thought that it was only because the air was warmer and more humid but I have been in warm and humid places before (not in a cave though), and they haven't caused shortness of breath or nausea. I don't remember any abnormal sweating. We continued the passage about 50 metres (hard to say exactly how long), and breathing got harder as we advanced. 

Received this reply from a very experienced Canarian-based caver familiar with San Marcos: "I thought you had perhaps enquired about some kind of scientific research when you asked about air quality... your colleagues have no reason to worry, San Marcos is perfectly safe, my fellows even teach caving courses there. Most probably it is elevated humidity and warmer caves being misinterpreted".

But you made the smart move! 
Get out, then anylize the problem from the safety of fresh air.  Well done.

Thank you all. Whatever causes nausea and shortness of breath, it's always a good move to turn back when you don't feel well. Caving should be fun, and it's not fun when you are feeling unwell. I have read that bad air causes same kind of symptoms, so that was my hypothesis after our experience. The hands-and-knees crawl and that boulder climb straight after it obviously slows down the air current, and air up in the big passage therefore becomes warmer and more humid. Interesting anyway.

Visited last week and didn't enter any bad air so it must have sorted itself out.

Maybe ;-)