The famous Dr Malthus thought that population would multiply faster than food production could grow. He was not primarily concerned with the finite size of the world but that, by increasing more rapidly than food supplies, the population would condemn itself to starvation. It is often said that his gloomy prediction was wrong; but was it? Today there are more undernourished people in the world than there were people in the world altogether in Malthus’s day, and unless we are ready to magnify the misery and create perpetual injustices, the growth of population must be urgently restrained. This is so close to the heart of our problem that in discussing pollution, and air pollution in particular, we must accept it as a starting point. No arguments about it will be entered into here except to say that it is no solution to show that a certain very large population, of say, ten thousand million, could be maintained on the earth. People will not tolerate indefinitely a kind of battery-farm existence, with all chemical necessities provided by benevolent technocrats. That is no one’s ideal.