Consider the following statements:

1.    Dobson unit is used for measuring column of stratospheric Ozone

2.    Highest levels of ozone are usually found in the mid to high latitudes, in Canada and Siberia (360 DU).

3.    When stratospheric ozone falls below 200 DU this is considered low enough to represent the beginnings of an ozone hole.

4.    Ozone holes commonly form during spring time above equator

Which of the statements is correct?