The Biden administration on Friday made final its long-awaited plan to offer billions of dollars in tax credits to companies that make hydrogen, in the hopes of building up a new industry that might help fight climate change.

When burned, hydrogen mainly emits water vapor, and it could be used instead of fossil fuels to make steel or fertilizer or to power large trucks or ships.

But whether or not hydrogen is good for the climate depends on how it is made. Today, most hydrogen is produced from natural gas in a process that emits a lot of planet-warming carbon dioxide. The Biden administration wants to encourage companies to make so-called clean hydrogen by using wind, solar or other low-emission sources of electricity.

In 2022, Congress approved a lucrative tax credit for companies that make clean hydrogen. But the Treasury Department needed to issue rules to clarify what, exactly, companies had to do to claim that credit. The agency released proposed guidance in 2023 but many businesses have been waiting for the final rules before making investments.

The final guidelines that were released Friday followed months of intense lobbying from lawmakers, industry representatives and environmental groups and roughly 30,000 public comments. They include changes that make it somewhat easier for hydrogen producers to claim the tax credits, which could total tens of billions of dollars over the next decade.

“Clean hydrogen can play a critical role decarbonizing multiple sectors across our economy, from industry to transportation, from energy storage to much more,” said David Turk, the deputy secretary of energy. “The final rules announced today set us on a path to accelerate deployment.”

Initially, Treasury had imposed strict conditions on hydrogen subsidies: Companies could claim the tax credit if they used low-carbon electricity from newly built sources like wind or solar power to run a machine called an electrolyzer that can split water into hydrogen and oxygen. Starting in 2028, those electrolyzers would have to run during the same hours that the wind or solar farms were operating.

Without those conditions, researchers had warned, electrolyzers might draw vast amounts of power from existing electric grids and drive a spike in greenhouse gas emissions if coal- or gas-fired power plants had to run more often to meet the demand.

Yet many industry groups and lawmakers in Congress complained that the proposed rules were so stringent, they could throttle America’s nascent hydrogen industry before it even got going.

Among the concerns: The technology to match hydrogen production with hourly fluctuations in wind and solar power is still in its infancy. Owners of nuclear reactors also said that they had been left out.

So the final rules contain several significant tweaks:

• Hydrogen producers will get two extra years — until 2030 — before they are required to buy clean electricity on an hourly basis to match their output. Until then, they can use a looser annual standard and still claim the tax credit.

• In certain states that require utilities to use more low-carbon electricity each year, hydrogen producers will now have an easier time claiming the credit, on the theory that those laws will prevent a spike in emissions. For now, Treasury said, only California and Washington meet this criterion, but other states could qualify in the future.

• Under certain conditions, companies that own nuclear reactors that are set to be retired for economic reasons can now claim the credit to produce hydrogen if it would help the plants stay open. Existing reactors that are profitable would not be able to claim the credit.

• The final rules also lay out criteria under which companies could use methane gas from landfills, farms or coal mines to produce hydrogen — if, for instance, the methane would have otherwise been emitted into the atmosphere.

The guidelines “incorporate helpful feedback from companies planning investments,” said Wally Adeyemo, the deputy Treasury secretary.

Some hydrogen producers said that many, though not all, of their biggest concerns had been addressed in the final guidance, which runs nearly 400 pages.

“There’s a degree of relief that the rules are, on balance, an improvement from the original draft,” said Frank Wolak, chief executive of the Fuel Cell and Hydrogen Energy Association, a trade group. “But there’s a lot in the details that needs to be evaluated.”

The lack of clear guidance had been holding up investment, said Jacob Susman, chief executive of Ambient Fuels, a clean hydrogen developer that is planning roughly $3 billion in projects across the United States. “Now that we actually have something solid, we can get down to the business of building,” he said.

Environmentalists said that most of the safeguards in the original proposal to prevent emissions from surging had been kept in place.

“The extra flexibilities granted to the green hydrogen industry are not perfect from a climate perspective,” said Erik Kamrath at the Natural Resources Defense Council. “But the rule maintains key protections that minimize dangerous air and climate pollution from electrolytic hydrogen production.”

The Energy Department estimates that the use of cleaner forms of hydrogen could grow to 10 million tons per year by 2030, up from virtually nothing today.

But political uncertainty looms. A new Congress could repeal the tax credits, although hydrogen generally enjoys support from both Democrats and Republicans and a number of oil and gas companies have invested in hydrogen technologies. The Trump administration could also revise the rules around the credits, although that could take years.

Economics are another hurdle. Producing cleaner hydrogen still costs $3 to $11 per kilogram, according to data from BloombergNEF. By contrast, it costs about $1 to $2 per kilogram to make hydrogen from natural gas.

The new tax credit will be worth up to $3 per kilogram, which could bridge the gap in some cases but not all. Technology costs would have to decline sharply.

Even with hefty subsidies to produce hydrogen, it’s not clear that enough buyers will emerge. Around the world, hydrogen companies have canceled several major projects over the last few years because of lack of demand. Steel makers and electric utilities that might have interest in the fuel often balk at the costly equipment required to use it.

“These new rules will probably help, even if they don’t go as far as many in industry wanted,” said Aaron Bergman, a fellow at Resources for the Future, a nonpartisan Washington research organization. “But there’s still the challenge of finding the people to consume the hydrogen you produce.”

Alcohol is a leading preventable cause of cancer, and alcoholic beverages should carry a warning label as packs of cigarettes do, the U.S. surgeon general said on Friday.

It is the latest salvo in a fierce debate about the risks and benefits of moderate drinking as the influential U.S. Dietary Guidelines for Americans are about to be updated. For decades, moderate drinking was said to help prevent heart attacks and strokes.

That perception has been embedded in the dietary advice given to Americans. But growing research has linked drinking, sometimes even within the recommended limits, to various types of cancer.

Labels currently affixed to bottles and cans of alcoholic beverages warn about drinking while pregnant or before driving and operating other machinery, and about general “health risks.”

But alcohol directly contributes to 100,000 cancer cases and 20,000 related deaths each year, the surgeon general, Dr. Vivek Murthy, said.

He called for updating the labels to include a heightened risk of breast cancer, colon cancer and at least five other malignancies now linked by scientific studies to alcohol consumption.

“Many people out there assume that as long as they’re drinking at the limits or below the limits of current guidelines of one a day for women and two for men, that there is no risk to their health or well-being,” Dr. Murthy said in an interview.

“The data does not bear that out for cancer risk.”

Only Congress can mandate new warning labels of the sort Dr. Murthy recommended, and it’s not clear that the incoming administration would support the change.

Still, President-elect Donald J. Trump does not drink, and his choice to head the Health and Human Services Department, Robert F. Kennedy Jr., swore off alcohol and drugs decades ago, and says he regularly attends AA meetings.

There is no question that heavy consumption is harmful. But supporters of moderate drinking — including makers of wine, beer and spirits, and some physicians and scientists — argue that a little alcohol each day may reduce cardiovascular disease, the No. 1 killer in the United States.

Newer scientific studies have criticized the methodology of earlier studies, however, and have challenged that view, which was once a consensus.

While most cancer deaths occur at drinking levels that exceed the current recommended dietary guidelines, the risk for cancers of the breast, the mouth and the throat may rise with consumption of as little as one drink a day, or even less, Dr. Murthy said on Friday.

Overall, one of every six breast cancer cases is attributable to alcohol consumption, Dr. Murthy said. More recent studies have also linked moderate alcohol consumption to certain forms of heart disease, including atrial fibrillation, a heart arrhythmia.

Two scientific reviews will be used to inform the updated recommendations about alcohol consumption in the federal dietary guidelines.

Five years ago, the scientific report that informed the writing of the 2020-2025 dietary guidelines acknowledged that alcohol is a carcinogen and generally unhealthy and suggested “tightening guidelines” by capping the recommendation for men at one standard drink, or 14 grams of alcohol a day.

When the final guidelines were drafted, however, there was no change in the advice that moderate drinking of up to two drinks a day for men was acceptable.

But the government acknowledged emerging evidence indicating that “even drinking within the recommended limits may increase the overall risk of death from various causes, such as from several types of cancer and some forms of cardiovascular disease.”

Since then, even more studies have linked alcoholic beverages to cancer. Yet any attempt to change the warning labels on alcoholic beverages is likely to face an uphill battle.

The current warning label has not been changed since it was adopted in 1988, even though the link between alcohol and breast cancer has been known for decades.

It was first mentioned in the 2000 U.S. Dietary Guidelines. In 2016, the surgeon general’s report on alcohol, drugs and health linked alcohol misuse to seven different types of cancer.

More recently, a scientific review of the research on moderate drinking, carried out under the auspices of the National Academies of Sciences, Engineering and Medicine, was commissioned by Congress.

That analysis found a link between alcohol consumption and a slight increase in breast cancer, but no clear link to any other cancers. The report also revived the theory that moderate drinking is linked to fewer heart attack and stroke deaths, and fewer deaths overall, compared with never drinking.

The World Health Organization says there is no safe limit for alcohol consumption, however, and 47 nations require warnings on alcoholic beverages. But cancer is rarely mentioned.

To date, only South Korea has a label warning about liver cancer, though manufacturers can choose alternative labels that don’t mention cancer. Ireland is currently slated to introduce labels that say there is a “direct link between alcohol and fatal cancers” in 2026.

The industry has a strong history of fighting warning labels that mention cancer, and alcohol-producing nations have also challenged warning labels under international trade law.

Industry opposition led to the premature termination of a federally funded Canadian study of the impact of warning labels that mentioned cancer.

The surgeon general’s advisory provided a brief overview of research studies and reviews published in the past two decades, including a global study of 195 countries and territories involving 28 million people.

They all found that higher levels of alcohol consumption were associated with a greater risk of cancer.

Other studies looked at specific cancers, like breast cancer and mouth cancer, finding the risks increased by 10 percent and 40 percent, respectively, for those who had just one drink a day, when compared with those who did not drink.

The report described the biological mechanisms by which alcohol is known to induce cancerous changes at the cellular level.

The most widely accepted theory is that inside the body, alcohol breaks down into acetaldehyde, a metabolite that binds to DNA and damages it, allowing a cell to start growing uncontrollably and creating a malignant tumor.

Animal experiments have shown that rodents whose drinking water was spiked with either ethanol, the alcohol used in alcoholic beverages, or with acetaldehyde developed large numbers of tumors all over their bodies.

Research has shown that alcohol generates oxidative stress, which increases inflammation and can damage DNA.

It also alters levels of hormones like estrogen, which can play a role in breast cancer development, and makes it easier for carcinogens like tobacco smoke particles to be absorbed into the body, increasing susceptibility to cancers of the mouth and the throat.

The surgeon general’s report also goes into detail about the increase in risk associated with drinking, differentiating between the increases in absolute risk and in relative risk.

For example, the absolute risk of breast cancer over a woman’s life span is about 11.3 percent (11 out of 100) for those who have less than a drink a week.

The risk increases to 13.1 percent (13 of 100 individuals) at one drink a day, and up to 15.3 percent (15 of 100) at two drinks per day.

For men, the absolute risk of developing an alcohol-related cancer increases from about 10 percent (10 of every 100 individuals) for those who consume less than one drink a week to 11.4 percent (11 per 100) for those who have a drink every day on average. It rises to 13 percent (13 of 100 individuals) for those who have two drinks a day on average.

Many Americans don’t know there is a link between alcohol and cancer.

Fewer than half of Americans identified alcohol use as a risk factor for cancer, compared with 89 percent who recognized tobacco as a carcinogen, according to a 2019 survey of U.S. adults aged 18 and older carried out by the American Institute for Cancer Research.

Yet alcohol consumption is the third leading preventable cause of cancer, after tobacco and obesity, according to the surgeon general’s report.

Dr. Murthy said it was important to know that the risk rises as alcohol consumption increases. But each individual’s risk of cancer is different, depending on family history, genetic makeup and environmental exposures.

“I wish we had a magic cutoff we could tell people is safe,” he said. “What we do know is that less is better when it comes to reducing your cancer risk.”

“If an individual drinks occasionally for special events, or if you’re drinking a drink or two a week, your risk is likely to be significantly less than if you’re drinking every day,” he added.