Motor vehicles and guns have far overtaken faulty machinery or chemicals as the major threats to the lives and health of American workers on the job, according to new findings.

The new dangers are being "virtually ignored" by employers and government regulators, who still worry mainly about manufacturing injuries that cause no more than one-seventh of all work-related deaths, the study says.

These are among the conclusions of what is probably the most painstaking study of job deaths ever conducted, a Johns Hopkins examination of 148 deaths, every work-related fatality in Maryland in 1978.

Another conclusion is that the switch to small cars is putting drivers, including those who drive on the job, at growing peril. The increasing switch of employers to small-car fleets can "double a worker's likelihood of being killed in a job-related crash," say the study's authors, Prof. Susan Baker and colleagues of the Johns Hopkins University School of Public Health.

The study's results appear in today's Journal of the American Medical Association and were presented at a Capitol Hill news conference yesterday by Johns Hopkins officials and the Insurance Institute for Highway Safety, a sponsor of the study.

Baker said several "simple steps," including redesign of truck cabs and wheels and stronger but not necessarily costlier auto bodies, could sharply cut the death toll.

Sen. John C. Danforth (R-Mo.), chairman of a Senate surface transportation subcommittee, called the findings astounding, and the federal government's "insensitivity to the slaughter" inexcusable. He said he will ask the National Highway Traffic Safety Administration and Occupational Safety and Health Administration why even remedial steps have been ignored.

The study was the first in any state to seek out and analyze every job-related death. In part, Baker said, this is because there are no central registries of such deaths, so no one usually knows all the causes.

In Maryland, it was learned, vehicles -- primarily trucks and cars but also boats, planes and non-road machines like cranes, forklifts and bulldozers -- were involved in 57 percent of the 148 deaths.

Firearms, primarily handguns, were the second leading cause, figuring in 11 percent of the deaths, and showing the growing danger of shooting to taxi drivers, clerks, police and others.

There were also deaths from the traditional causes of industrial accidents: dangerous chemicals or burning fluids, electrical accidents and cave-ins.

But throughout the nation, the authors said: "As the hazards of earlier years have been reduced, new problems have taken their place . . . . Yet) safety literature, regulations and training programs continue to emphasize injuries in manufacturing . . . where only 14 percent of all work-related deaths occur . . . . Most occupational safety programs virtually ignore transportation vehicles."

The U.S. Department of Transportation has given little attention, they said, to protecting truck drivers and passengers, though heavy trucks, mainly tractor-trailers, were involved in nearly two-thirds of the road vehicle deaths.

Baker recommended many steps, including alterations in trailer-truck design that give drivers as much protection as is provided for their cargoes.

On employers' growing use of unsafe small cars for economy, she said, "Can you imagine these same employers taking the guards off the machines because it would be cheaper? OSHA certainly would not allow it. The workers, the unions certainly would not."