Most of us are moved by compassion and generosity when confronted with what one United Nations agency calls ''loud emergencies'' - famine, floods, earthquakes.
Yet we tend to be less urgently aware of the ''silent'' emergencies: the 45, 000 children who die in the world every day from preventable diseases, the 400 million children that UNICEF says go to bed hungry each night, the up to 1 billion growing up illiterate.
One of the most challenging ''silent'' emergencies of all is overpopulation in the poorer two-thirds of the world, where about 73 million people - the equivalent of another Mexico - are being added to already jammed and poor slums and villages every year. It's a crisis that creeps rather than careers. It will take seven or so years before babies born today need schooling, and another 10 or so before they have children of their own. It's gradual, but it's cumulative.
Behind the statistics, though, are human drama and urgency: the squalor of Calcutta and Bombay slums, the garbage dump scavengers of Mexico City, empty food shelves in Rio and Dacca and Bangkok, lack of jobs, overflowing schoolrooms , food riots.
It can be argued that the greatest immediate threat to world peace is not the nuclear bomb, but rather the increasingly desperate third-world battle for a share of such basic resources as land, food, water, housing, clean air, jobs, and health care. By the year 2000, just 16 years from now, the third world will contain almost 5 billion people, more than the whole world holds today (4.6 billion). The UN forecasts that global population, at current growth rates, will be 6.1 billion by 2000.
Unless more voluntary family planning information is made available, the UN believes, areas of Africa simply won't be able to carry excess numbers, and war or starvation will follow.
If human need alone isn't galvanizing enough, consider that the United States has strategic interests all over the third world, from oil to rubber to tin to naval routes to military bases. Growing poverty and overcrowding cannot work in anyone's interests except perhaps those of the Soviet bloc.
One convert to a need to awaken attention to the ''silent'' population emergency is Pranay Gupte. Born in Bombay, educated at Brandeis University, he was a New York Times Africa correspondent based in Nairobi.
In the late summer of 1982, having left the Times, Gupte began 14 months of travel through 38 countries, some rich but most poor. He wanted to write a book to mark the 1984 World Population Conference in Mexico City (held Aug. 6-14) because, he says, overcrowding and excessive growth rates were among the deeper trends beneath the daily headlines he had pursued for so long.
The book has just appeared. Its virtue is that it tries to concentrate on what overcrowding means to individual people in the third world. It looks at the human side while presenting the demographic essentials as well.
It is a highly personal account of his travels: a journalist's book, filled with observation and anecdote. The writing is chatty and enthusiastic, at times very much so. Some chapters could perhaps have been shorter.
Despite his cheerful style, he ends his journey on a pessimistic note. Having seen overcrowding with his own eyes, he writes, ''I don't think the (population) bomb has been defused at all. It's fuse is long, and it is still burning.'' Because of its timing and its approach, the book also forms part of the political debate that has broken out in Washington over population.
Gupte, who had a travel grant from the UN Fund for Population Activities (UNFPA), came face to face with overcrowding. Others, less well traveled, see from their geographical and moral vantage point in the US much less to worry about. A number of anti-abortion ''right-to-life'' lobbyists seem hardly to see a population crisis at all. Ultimately they want to remove US family planning aid from any country where abortion is legal. For them, the overriding moral issue is avoiding abortion. Advocates of voluntary family planning, such as Mr. Gupte, see the chief moral point in avoiding countless, unwanted, unloved children without prospects or hope.
Pick up this book and dip into it. Read the excellent foreword by UNICEF official and writer Varindra Tarzie Vittachi. Read more widely, make up your own mind on the issue - and act accordingly.