As demand for meat and dairy products increases across the world, much attention has landed on how livestock affect the environment, from land usage to greenhouse gas emissions.

Researchers at the Georgia Institute of Technology and the Centers for Disease Control & Prevention are highlighting yet another effect from animals raised for food and their consumers: the waste they all leave behind.

In a paper published Nov. 13 in Nature Sustainability, the researchers put forth what they believe is the first global estimate of annual recoverable human and animal fecal biomass. In 2014, the most recent year with data, the number was 4.3 billion tons and growing, and waste from livestock outweighed that from people five to one at the country level.

"Exposure to both human and animal waste represent a threat to public health, particularly in low-income areas of the world that may not have resources to implement the best management and sanitation practices," said Joe Brown, an assistant professor in Georgia Tech's School of Civil & Environmental Engineering. "Estimating the amount of recoverable feces in the world also highlights the enormous potential from a resource perspective."

Metals, phosphorus, nitrogen and potassium are all among the resources that could be recovered from human and animal waste, Georgia Tech said. The researchers pointed to an earlier analysis estimating that the value of recoverable metals alone from the waste of 1 million people reaches $13 million a year.

The researchers looked at data from 2003 to 2014 as well as projections through 2030. The study combined global animal population data from the U.N., human population data from World Bank as well as earlier research on animal-specific estimates of fecal production.

From 2003 to 2014, the amount of waste biomatter produced grew annually by more than 57 million tons as both human and livestock populations grew, Georgia Tech said. The researchers estimate that by 2030, the total amount of global fecal biomass produced each year could reach at least 5 billion tons, with livestock waste outweighing human waste six to one at the country level.

"This paper demonstrates that building more latrines in developing parts of the world isn't going to solve all of our waste management problems," Brown said. "Animal waste has the potential to negatively impact health in many of the same ways as with human waste, from spreading enteric infections to hurting growth and cognitive development of the humans exposed."

While chickens are the most plentiful livestock globally, cattle produce the most fecal waste on the planet due to their larger body mass. As a result, countries with high numbers of cattle, such as those in the Americas, produce the most waste by mass.

The researchers estimate that by 2030, the planet's total annual fecal and urinary biomass could contain up to 100 million tons of phosphorus, 30 million tons of potassium, 18 million tons of calcium and 5.5 million tons of magnesium, to name a few recoverable materials.

While much of the attention on reducing disease transmission through the decades has focused on pathogens associated with human waste, much less attention has been given to animal waste, despite livestock accounting for 80% of the global fecal biomass generated, according to the researchers.

"Ultimately, shining a light on the amount of waste that we produce is the first step toward shaping policies and regional planning geared toward maximizing public health and resource recovery," Brown said. "This is an area where there's a huge need for attention and investment — to help develop next-generation waste management innovations for both large-scale and small-scale animal husbandry operations that will enable us to maximize human health and meet the global demand for natural resources."