Even a bullet, fired straight up at the maximum speed a gunpowder blast can accelerate it to; will never leave the lowest layer of Earth's atmosphere. A combination of gravity and air resistance will slow it down until it reaches a maximum height, whereupon it will fall back down to Earth's surface. Atmospheric oxygen levels are very slowly decreasing today due to the burning of fossil fuels, which consumes oxygen, and deforestation which reduces oxygen production, but not enough to alter biological processes.