The World Health Organization last week reported that 47 percent of the premature death in the world could be averted if 20 hazards to health -- everything from risky sexual practices to insufficient zinc in food -- could be eliminated.

That estimate suggests that even after a century of virtually unchecked rise in life expectancy around the world, there's still huge room for improvement. But how realistic is that idea? How easy is it to modify "modifiable risks" on a gigantic scale?

Those are the obvious questions that government officials and public health experts will ask themselves when they review the 248-page document.

The report is more than an audacious effort to quantify what people die of, and why. It's also supposed to provide a rough map through the supermarket of disease-prevention options, where things come in all prices, sizes and availability.

The WHO team of researchers set a target level for all of the risk factors. In some cases, this was the absence of the hazard (such as no iron deficiency or no smoking); in others, it was a specified number (such as a systolic blood pressure of 115 millimeters of mercury). Removing the risks entirely is an unrealistic goal under any circumstance. Nevertheless, it gives an idea of the magnitude of benefit.

Globally, if the 20 leading risks were driven to a "theoretical minimum," life expectancy today would be about 10 years longer than it is. In sub-Saharan Africa (where life expectancy is falling because of the AIDS epidemic), the gain would be 16 years. In Japan, which has the longest life expectancy, four more years could be bought.

Much can be gained by making more realistic inroads on risks responsible for large numbers of premature death. Reducing malnutrition in children by 25 percent and reducing unsafe sexual behaviors by a similar amount would eliminate 5 percent of the world's burden of disease by 2010. Nearly all of that would come from the increased survival of African children, and prevention of new HIV infections.

The world's risk profile, of course, isn't static. Even without active intervention, it changes.

Early death attributable to childhood malnutrition fell from 16 percent in 1990 to 9.5 percent in 2000. This continues a trend observed for decades -- and which, curiously, hasn't stopped despite little reduction in global poverty in the last 10 years. Christopher J.L. Murray, the WHO epidemiologist who co-led the recent project, speculates that higher educational attainment of girls may be the reason. That has given young mothers more power and autonomy in households, which is highly beneficial to their children even if the food supply remains inadequate.

By the same token, some causes of premature death are growing. Being overweight and obese today are responsible for 2.5 percent of lost years around the world. This will increase by about one-third by 2020 if steps to slow it aren't instituted. Similarly, premature death from AIDS will increase enormously in the absence of action.

Epidemiologists recognize two general strategies of disease prevention. (A third may also be emerging.) Which is more attractive or palatable depends on the disease being targeted, and also on political philosophy and money.

The first puts great emphasis on population-wide interventions intended to reach everyone regardless of individual risk for a disease. Laws, economic incentives and government action are essential for making this happen.

The alternative strategy targets individual people with elevated risk for a disease. This is the one generally preferred in places such as the United States, where scientific diagnosis, tailored medical care and personal choice are highly valued.

The WHO researchers -- more than a hundred epidemiologists and biostatisticians from around the world -- found worthwhile interventions of both types.

For example, fortification of food with iron (in cereal flours or noodles), zinc (in wheat products) and vitamin A (in sugar) is cost-effective even for the poorest countries, because it would greatly reduce death from diarrhea, malaria, measles and pneumonia in children. Recommended population-wide interventions, however, aren't limited to micronutrients or the diseases of the poor.

Dietary salt contributes to high blood pressure, a top-10 risk factor for early death everywhere in the world. Regional strategies to reduce salt in processed food have been tried in Australia.

Studies show that when guidelines for food manufacturers are voluntary, dietary sodium intake falls about 15 percent. When they are mandatory, it falls about 30 percent.

Similarly, mass marketing campaigns to reduce dietary fat lead to about a 2 percent reduction in average cholesterol level over a decade. For an individual that's a small change, but in a population it leads to a significant drop in cardiovascular mortality. That's because in most places, nearly everyone has cholesterol above the truly "safe" level. Consequently, when a population's average cholesterol falls, it means that nearly everyone has cut his risk, not just the people with the highest levels.

The more common prevention strategy in the developed world, however, is to look for people with increased risk, and aim preventive measures only at them. The WHO team found this also works. High blood pressure is enough of a problem, it noted, that screening adults for hypertension and treating everyone with a systolic blood pressure over 160 with a cheap diuretic drug is cost-effective even in the poorest countries.

A third way, however, may offer the best approach to cardiovascular disease, which is the leading cause of death in rich countries and an important one everywhere else. It combines aspects of both population-wide and individually targeted prevention.

Every individual's risk would be evaluated on the basis of age, sex, a few facts of personal medical history, a blood pressure measurement and possibly also a cholesterol measurement. If the profile revealed risk above a specified threshold -- say, that a person had a 25 percent chance of suffering a heart attack or stroke in the ensuing decade -- he or she would then be prescribed a one-size-fits-all treatment.

Sometimes referred to as a "risk biscuit," the treatment would consist of fixed doses of generic drugs -- a diuretic (which lowers blood pressure); a beta blocker (which does likewise and also protects the heart); a statin (which reduces cholesterol); and aspirin (which thins the blood).

It doesn't matter whether the person needs every component in a formal sense. Each drug reduces risk for most people, and that's close enough. And with a price of $14 a year (by Murray's calculation), the combination is within reach of hundreds of millions of people.