Final answer:
Pollen is essential for plant sexual reproduction but can also be an air pollutant. Wind-pollinated plants release large amounts of pollen into the air, contributing to allergies and respiratory issues in humans, which classifies it as a pollutant.
Step-by-step explanation:
Pollen grains are crucial for the sexual reproduction of seed plants, serving as the male gametophytes that carry sperm. During reproduction, pollen is transferred from the anther to the stigma of a flower, which enables fertilization and the creation of seeds. While this process is vital for the continuation of plant species, pollen can also become an air pollutant. This occurs particularly in wind-pollinated plants like grasses, oaks, and birches, which produce large amounts of pollen that can be carried over great distances.
Conversely, animal-pollinated flowers have evolved to attract specific pollinators and typically produce less airborne pollen. When pollen is present in high concentrations in the air, it can cause allergic reactions in sensitive individuals, leading to conditions like hay fever, and thus, becomes considered an air pollutant.