A recent study may give germophobic students yet another reason to dread going to lecture.

A joint study of indoor microbial composition by UC Berkeley and Yale University researchers found that human presence causes a significant increase in levels of bacteria and fungi indoors. The average human emission, the study states, is about 37 million bacteria per person per hour.

The study — entitled “Size-resolved emission rates of airborne bacteria and fungi in an occupied classroom” — was conducted by collecting and comparing air samples from empty classrooms, occupied classrooms and the outdoors. In samples from occupied classrooms, researchers found elevated levels of microbes shed from human skin and cavities and kicked up in floor dust.

“Whenever you’re in a densely populated space, you’re breathing bacteria coming from other people,” said William Nazaroff, co-author of the study and campus professor of civil and environmental engineering.

The study was originally published online Feb. 13 in Indoor Air, but it has recently garnered attention because of an article written about it March 29 in Discovery News.

According to Nazaroff, the study involved state-of-the-art experimental techniques, including DNA analysis of microbes and particulates. The result was one of the most complete analyses of indoor microbial air composition to date. Previous studies of microbial presence in the air involved culturing — growing captured particulates — and identifying what was able to grow. The problem, Nazaroff said, was that not all microbes could be grown, and thus could not be identified.

With modern techniques, the study for the first time was able to quantify exactly how many bacteria humans were responsible for emitting.

Despite the somewhat unsettling nature of the study’s findings, the impact on human health is still uncertain, Nazaroff said.

“Historically, people have worried about bacteria in buildings because of airborne disease transmission,” he said. “But there may be a benefit to certain kinds of these bacteria.”

Gary Andersen, head of the Ecology Department at Lawrence Berkeley National Laboratory, emphasized the importance of this new procedure of microbial DNA analysis to studying an indoor environment.

“I think it’ll help for public health and for building design to take into consideration how microbes can impact human health,” he said.

Next, the researchers plan to carry out similar testing in university housing and elementary schools around the world.