The earth before the flood had lush tropical forests and large amounts of vegetation. The days were longer and the temperature was warmer. The general climate acted as a shield which kept out more harmful radiation from reaching the earth's surface than it does today.
There are many factors that could account for the lowering of the life spans after the flood. The Bible states that the flood would not only destroy the land dwelling air breathing animals but it would also destroy the earth (Genesis 6:13; 9:11). After the flood the earth was completely different than the earth before. There were widespread global differences. These would include changes in the climate, composition of the atmosphere, hydrologic cycle, geologic features, cosmic radiation reaching the earth, ozone concentration, ultra violet light, background radiation, genetics, diet, coupled with a host of other subtle and profound chemical and physiological changes. These changes caused a rapid decline of the longevity of post-flood humanity.
The average life span before the flood (ten generations) was 857 years. After the flood, however, the average life span (for the ten generations from Shem to Abraham) was only 317 years! In 2008 the average life expectancy of a person living in the United States was 78 years.