H e spent that first long day ministering to the injured and preparing for what he thought would be a major demand for ambulances and triage workers.

But by the evening of Sept. 11, 2001, Robert Lahita, a physician from Ridgewood, N.J., and chief of Hudson County Emergency Medical Services, learned what we all would learn: The number of injured was surprisingly small for so large a catastrophe.

The number of those beyond help, the dead, however, was, to use Mayor Rudy Giuliani’s word, “unbearable.”

Still, Lahita — then on the staff of St. Vincent’s in Lower Manhattan — felt he had to do something. So, with a large box of surgical masks, he boarded a boat in Jersey City, N.J., and crossed the Hudson. I went with him.

The boat was jammed with men in hard hats and sweatshirts with sleeves cut back to show muscular arms. They wore heavy leather tool belts. Steelworkers, taking time from building the Goldman Sachs tower in Jersey City to help at Ground Zero.

It was a time when virtually anyone could just show up and say they wanted to help. When the need to do something, anything, was compelling, and the normal, common-sense rules for keeping people away from a disaster scene were forgotten.

“They were very sincere and patriotic and I was very impressed with them,” Lahita says. He now is chief medical officer at the Liberty Health Center in Jersey City.

Impressed, but worried. More than 24 hours after the towers fell, Lower Manhattan was still blurred in a fog of that odd, yellowish-brown dust that covered the ground to the depth of several inches near Ground Zero.

Powdered glass. Concrete. Marble. Steel. Asbestos. All manner of material, literally pulverized. When we walked through the stuff, it kicked up into the air, like the loose, dry snow that rises up, swirling and ghostly, from a highway in the first few minutes of a blizzard.

Lahita insisted that I wear a mask, and he wore one. Every person he passed, he pulled out a new mask from the box and gave it out, insisting it be worn. We were stunned to see so many people on the rubble pile — including cops and those steelworkers — without them.

“Those kinds of masks weren’t much help,” he remembers now, “but they were better than nothing.”

Lahita wore his county medical services uniform that day. Official, impressive, with gold stars on his collar. His authority was limited, his mission voluntary. A concerned doctor trying to help. But he looked like brass.

A man approached him. He wore civilian clothes and, even on a day when everyone’s expression was grim and frightened, his face reflected even more serious concern.

“Are you in charge?” the man asked Lahita.

“No, why?”

“These people have to get out of here. What they’re breathing is going to kill them some day.”

The man, who said he was from the federal Environmental Protection Agency, gestured to the police officers and volunteers working around him.

“No one but the firemen have proper masks,” he said. He looked at Lahita’s box of surgical masks. “They won’t do any good.”

The man walked off in search of someone with more authority. Lahita went to a command center in a trailer and reported what he heard. But, in the first day after such a frightening, unprecedented attack, what rescue workers were breathing did not seem much of a priority then.

Later, the EPA, then run by former New Jersey Gov. Christine Todd Whitman, issued reports indicating the air in Lower Manhattan was safe to breathe.

“I remembered the guy at Ground Zero and I was surprised,” Lahita recalls, “but I figured they had the science to prove what they were saying.”

A study conducted by Mount Sinai Hospital in New York released Thursday suggests that many of those working at the site after the attacks got sick or are sick, probably because of what they breathed.

“That doesn’t surprise me,” Lahita says. “That’s what we thought all along. It just makes sense.”

Of course, because lawsuits have been filed, some will say the study doesn’t prove anything. New York’s mayor has expressed his doubts.

Maybe. Still, this week, as we remember the dead, those who can no longer be helped, it might be the most fitting observance of this fifth anniversary of that unforgettable event to ensure we do not forget its living victims.