Complex sentence: Since neutrons are absorbed by air, neutron radiation effects drop off very rapidly with distance in air, there is a sharper distinction, as opposed to thermal effects, between areas of high lethality and areas with minimal radiation doses.
Split into simpler sentences: Since neutrons are absorbed by air, neutron radiation effects drop off very rapidly with distance in air. As such, there is a sharper distinction, as opposed to thermal effects, between areas of high lethality and areas with minimal radiation doses.