Complex sentence: Due to the small amount needed and the fast rate that temephos breaks down in water, this type of larvicide does not pose an unreasonable health risk to humans, but at large doses it can cause nausea or dizziness.
Split into simpler sentences: Due to the small amount needed and the fast rate that temephos breaks down in water, this type of larvicide doesn't pose an unreasonable health risk to humans. But at large doses it can cause nausea or dizziness.