Why is it difficult to breathe at high altitudes?

it’s hard to breath at high altitudes because the outside air pressure is lower than it is inside your lungs making it difficult to pull in the thinner air

Explanation:

the "dirty dozen" chemicals banned in an international treaty are formally known as persistent organic pollutants, or pops. the united nations environment program provided this background on what they are and what they can do to humans and wildlife.

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