Consider the following statements regarding air pollution: 
1) The pollutant caused by incomplete combustion of organic matter is carbon monoxide. 
2) Depletion of ozone in outer atmosphere may trigger skin . 
3) Acid rains are caused by SO₂ and NO_x. 
4) The permissible standard for SO₂ in air for residential areas in India is . 
Which of the above statements are correct?