Why in the lower layers of the atmosphere is currently reduced oxygen concentration

1) This is because the land cover of the Earth is reduced due to deforestation,
2) the death of the phytoplankton of the oceans due to its pollution,
3) oxygen consumption increases during the combustion of fuel (vehicles, industry, etc.).

Remember: The process of learning a person lasts a lifetime. The value of the same knowledge for different people may be different, it is determined by their individual characteristics and needs. Therefore, knowledge is always needed at any age and position.