Debris in orbits that nearly intersect the atmosphere are practically harmless. The issues arise when you have an orbit that will intersect other orbits a billion times before decay.

While the reality is very complicated, you can make a rough handwave model for orbital debris in a circular orbit: At 100km it lasts 1 orbit (90 minutes) at most, and every ~100km you add after that increases orbital lifespan by ~10x.