Millions of people across the nation work every day around materials that could prove dangerous or deadly without proper handling. But chemical accidents like the one that killed a worker in South Boston this week are rare, and have become rarer, as workplace safety improves.

According to federal data, deaths on the job have been falling for decades, and chemical exposure incidents — which make up a small percentage of such fatalities — have declined, as well.

In 2014, the most recent year of data available, 4,679 workers died on the job in the United States, an average of almost 13 deaths every day. That was down from about 38 worker deaths per day in 1970.

And of the 128,983 workplace deaths recorded nationwide since 1992, about 8.8 percent were attributed to “exposure to harmful substances or environments.’’

Only 0.03 percent were attributed to ammonia — which was leaking in the South Boston warehouse where a worker died Wednesday — as the main source of exposure, according to data from the Bureau of Labor Statistics.

The picture has been similar in Massachusetts, which has consistently had one of the nation’s lowest worker fatality rates in recent years, according to state by state analyses by the American Federation of Labor and Congress of Industrial Organizations.

“Most acute fatal injuries are not chemical exposure fatalities,’’ said a Boston University environmental health professor, Les Boden, who has researched occupational safety and health.

Such deaths have also become less common over the years, as have workplace deaths overall, because of new technology and heightened awareness, some of which was prompted by regulation, said BU environmental health professor Richard Clapp.

Most places where people work regularly with hazardous materials have equipment and other controls to prevent potential injuries or death from contamination.

However, experts believe that harmful chemicals and substances in the workplace are ultimately responsible for a significantly larger number of chronic illnesses and deaths that go untracked because the symptoms do not typically appear suddenly, but rather over the course of years.

For example, Clapp said, a type of cancer called mesothelioma is usually attributed to workplace asbestos exposure, but such deaths are not counted in workplace fatality statistics.

The most common type of workplace deaths in 2014 were from transportation-related incidents, which accounted for about 40 percent of deaths, followed by falls, slips and trips (17 percent); violence and other injuries by people or animals (16 percent); contact with objects and equipment (15 percent); and fires and explosions (3 percent).

Men accounted for 92 percent of workplace fatalities nationally in 2014.

Clapp called the South Boston case “an extreme situation.’’

“These things do happen, but they’re luckily rare,’’ he said.

Matt Rocheleau can be reached at matthew.rocheleau @globe.com. Follow him on Twitter @mrochele.