Is it dangerest if a tank of liquid nitrogen is leaking?

yes and no. The liquid nitrogen is not toxic but in its liquid form it is at (around) -330oF which will freeze and destroy flesh on contact. If you (for example) freeze a banana with it you can smash the fruit with a hammer and it will shatter like glass - so its effect on your body is extremely dangerous.

On the plus side it will very quickly return to a gaseous state

• because it will get above the boiling point very quickly at

normal room temperature (liquid nitrogen looks very much like boiling water if you have an open vessel). This could be dangerous in an unventilated room because you can't survive on nitrogen and you would, potentially, suffocate as it reduces the available oxygen in each breath.

As the leak will be obvious and, presumably, it is in an area where dealing with liquid nitrogen is a common occurrence so it will be ventilated and safety equipment is available the risk is small, however a leak of any vessel under pressure could lead to catastrophic failure especially because such temperatures can make metal brittle and the resulting explosive decompression could cause a lot of damage.