Demographers have developed powerful statistical tools for projecting population sizes, growth rates and composition. Growth is more difficult to slow down, however, because of the distinctive built-in momentum. Relatively simple health interventions, such as malaria control, oral rehydration therapy, or a vaccination programme can reduce infant mortality quickly. Total population remains similar for almost 30 years, since mortality is falling more rapidly in the community with more rapid fertility decline. Differences do not begin to appear for 25 years, but then they grow rapidly. It is common to build life tables from actual population conditions for different population groups, such as those for the developed countries, and those for various regions in the less developed world. To the extent that these regional differences reflect environmental differences, it can be said that environmental conditions inform population projections.