Some people seems to think so.... and newspapers seem to publish this nonsense:
Lets get the fact right this time:
Nitrogen, in our case is the molecular form of nitrogen N-N or N2 (as opposed to the Nitrogen atom) is found in our atmosphere as gas, and accounts for about 78% (source: http://www.space.com/17683-earth-atmosphere.html). Thanks to technological innovations we can now make Liquid Nitrogen (http://blogs.howstuffworks.com/2009/07/27/how-do-they-make-liquid-nitrogen/)/
The boiling temperature of liquid nitrogen is -196 centigrade. When it comes into contact with the water in the pool (roughly room temperature, say 25 degrees) it will boil pretty fast, creating lots of gaseous nitrogen. The nitrogen gas is still very cold and therefore cools the air around it. With all the humidity around (humidity is a measure of how much water molecules are in the air), the water condenses into droplets, just as if we were high up in the sky where its pretty cold. This is why you see all those clouds in the picture.
Here's what I had the pleasure of doing with liquid nitrogen:
So far it seems pretty harmless. HOWEVER... if the amount of nitrogen is so large, than it will displace (push) the air around the pool, which means that the people in the pool will have less oxygen to breathe, and are likely to pass out/go into a comma/die/drown....you get the point. The bottom line is that the danger is asphyxiation and not poisoning (as suggested in the article)
The moral of the story is that never stay in the pool when liquid nitrogen is thrown in. Outside is better, and be sure that this is an open space.
What about the chlorine you're wondering? Its still in the water. Nothing bad happened to it. Thanks for asking.
I would like to see more newspapers talk to actual chemists before printing such comments within their published work in the future. Save us a lot of headache explaining why the journalists got it wrong.