(CNN) The rate of people dying from cancer in the United States seems to have dropped steadily for 25 years, a new study says, but disparities remain between the rich and the poor.

The overall nationwide cancer death rate fell continuously from 1991 to 2016 by a total of 27%, according to a study by the American Cancer Society, published Tuesday in CA: A Cancer Journal for Clinicians

That translates to about 2.6 million fewer cancer deaths total than would have been expected if death rates stayed at their peak, which was seen in 1991, according to the study.

"The continued decline in the cancer death rate over the past 25 years is really good news and was a little bit of a surprise, only because the other leading causes of death in the US are starting to flatten. So we've been wondering if that's going to happen for cancer as well, but so far it hasn't," said Rebecca Siegel, first author of the study and strategic director of surveillance information at the American Cancer Society

According to the US Centers for Disease Control and Prevention , based on the latest data, the three leading causes of death in the United States in 2017 were heart disease, cancer and accidents or unintentional injuries.

Meanwhile, on a global scale, the number of people around the world who have cancer appears to be growing, according to the World Health Organization

A 'surprising' trend emerges

Some of the data date as far back to 1930, and the most recent data is from 2016.

The data showed that the nationwide cancer death rate climbed during most of the 20th century, largely driven by jumps in lung cancer deaths due to smoking and tobacco use

But since its peak of 215.1 deaths per 100,000 people in 1991, the cancer death rate dropped steadily by about 1.5% per year to 156 per 100,000 people in 2016, an overall decline of 27%.

The data also showed that a disparity in death rates between black and white cancer patients appeared to be closing.

"The racial gap in cancer mortality is continuing to narrow -- so it was that the cancer death rate in blacks was 33% higher than in whites in the mid-1990s, and the current data now indicate it's 14% higher -- so it's still higher, but the gap is narrowing, which is really good news," Siegel said.

However, the data also revealed a potentially troubling trend: a growing gap in death rates based on wealth.

"It was surprising to see that the disparities by socioeconomic status are actually widening," Siegel said. "Wealth causes differences in exposure to risk factors and also access to high-quality cancer prevention, early detection and treatment."

For instance, between 2012 and 2016, the overall cancer death rate was about 20% higher among people living in the poorest counties in the United States compared with those in the most affluent counties -- and socioeconomic inequalities in cancer mortality widened over the past three decades overall, according to the study.

Photos: Do you live near a cancer cluster? Photos: Do you live near a cancer cluster? A study shows cancer deaths in counties across the nation, revealing clusters that have lagged behind national cancer efforts. Deaths from all cancers in 2014 were highest along the Mississippi River, near the Kentucky-West Virginia border, western Alaska and the South in general. Deaths were lowest in places like Utah and Colorado. Hide Caption 1 of 11 Photos: Do you live near a cancer cluster? Deaths from colorectal cancers in 2014 were highest along parts of the Mississippi River, along the Kentucky-West Virginia border, and in Alaska, southern Alabama and Louisiana. Hide Caption 2 of 11 Photos: Do you live near a cancer cluster? Deaths from lung and airway cancers in 2014 were highest in Kentucky and West Virginia. Deaths were lowest in states such as Idaho, New Mexico, Utah, Colorado and parts of Arizona. Hide Caption 3 of 11 Photos: Do you live near a cancer cluster? Deaths from breast cancer in 2014 were highest along the Mississippi River and the Southern belt. Deaths were lowest in parts of the Northeast, West and Upper Midwest. Hide Caption 4 of 11 Photos: Do you live near a cancer cluster? Similar to breast cancer, deaths from prostate cancer in 2014 were highest along the Mississippi River and the Southern belt. Deaths were lowest in South Florida and along the US-Mexico border. Hide Caption 5 of 11 Photos: Do you live near a cancer cluster? Deaths from pancreatic cancer in 2014 were highest in the South and lowest in parts of Colorado, Arizona, Florida and Texas. Hide Caption 6 of 11 Photos: Do you live near a cancer cluster? Deaths from uterine cancer in 2014 were highest in parts of Montana and along the East Coast from South Carolina to Maine. Deaths were lowest in parts of Alaska, Texas and the Southwest. Hide Caption 7 of 11 Photos: Do you live near a cancer cluster? Deaths from kidney cancer in 2014 were highest in Alaska, Texas, Louisiana, Oklahoma and the Dakotas. Deaths were lowest in parts of Florida and Colorado. Hide Caption 8 of 11 Photos: Do you live near a cancer cluster? Deaths from liver cancers in 2014 were highest in Alaska, several counties in South Dakota, and parts of Texas along the border with Mexico. Deaths were lowest in the Midwest and mountain states. Hide Caption 9 of 11 Photos: Do you live near a cancer cluster? Deaths from testicular cancers in 2014 were highest in California and Nevada, with smaller clusters in Missouri, Michigan and Texas. Deaths were lowest in parts of Colorado, in the District of Columbia and around the Atlanta and Minneapolis areas. Hide Caption 10 of 11 Photos: Do you live near a cancer cluster? Deaths from non-Hodgkin's lymphoma in 2014 were highest in parts of the Midwest, the Appalachian region and Louisiana. Deaths were lowest near the "Four Corners" and in Alaska and western Texas. Hide Caption 11 of 11

The American Cancer Society also annually estimates the numbers of new cancer cases and cancer deaths that could occur nationwide, based on the most recent data.

This year, 1,762,450 new cancer cases and 606,880 cancer deaths are projected to occur in the United States. That corresponds to more than 4,800 new cases and almost 1,700 deaths per day, according to the study.

The study had some limitations, including that the projections should be interpreted with caution because they were based on data from three to four years ago.

Yet "it gives us a touch point on what's going on in cancer in the United States, because if we don't know what the trends are and the populations that are most burdened by the disease, then we can't do anything about it," Siegel said.

'A real critical issue in the cancer world'

The new study does a "very good job" summarizing those trends, said Dr. Walter Curran, executive director of the Winship Cancer Institute of Emory University in Atlanta, who was not involved in the study.

"The encouraging point is that cancer mortality continues to go down, particularly for men, but the tough part is, we're still seeing over 600,000 Americans dying of cancer every year," he said. "The good news is that we see fewer racial disparities in cancer mortality than in the past, but the bad news is, we see socioeconomic disparities in cancer mortality."

Dr. Dan Theodorescu, director of the Samuel Oschin Comprehensive Cancer Institute at Cedars-Sinai in Los Angeles, said he read the results of this annual study on cancer statistics each year.

"I would say that this is the best data out there for the oncology community and those concerned with healthcare in America," said Theodorescu, who was not involved in the study.

The reasons for the steady decline in cancer deaths could include nationwide reductions in smoking, improvements in detecting cancer and "revolutionary advances" in treating cancer, he said.

Still, disparities may persist because "socioeconomic status plays a pivotal role in cancer incidence and survival," Theodorescu said.

"Poverty has been a relentless obstacle to receiving cancer care because of lack of, or low insurance coverage. Getting to the oncologist often takes longer and options may be more limited," he said. "No insurance or low-coverage insurance also reduces the incentive to visit the doctor for symptoms and even more for preventive health practices, such as smoking cessation, yearly physicals, and immunizations against cancer-causing viruses."

Get CNN Health's weekly newsletter Sign up here to get The Results Are In with Dr. Sanjay Gupta every Tuesday from the CNN Health team.

Some of the counties with the highest poverty are in rural areas of south Georgia, and "this is something that we've become aware of here at the Winship Cancer Institute of Emory University," Curran said.

"What you see is a tragedy of increasing rates of obesity, which is now a risk for certain types of cancer; more clearly identified higher rates of tobacco use; and issues with access to cancer screening and prevention strategies and probably issues with access to diagnostic and therapeutic approaches to cancer," he added. "So it's a multi-factorial issue that we view as a real critical issue in the cancer world right now."