Air Pollution Inflames Brain, Lowering Sperm Counts: Mouse Study

The buildings of downtown Los Angeles are partially obscured in the late afternoon, as seen from Pasadena, Calif., on Nov. 5, 2019. Mario Tama/Getty Images

A recent paper has outlined one pathway that may explain the observed correlation between higher air pollution and lower sperm counts—namely, inflammation in particular neurons within a brain region known as the hypothalamus.

In the study, published on Sept. 8 in Environmental Health Perspectives, researchers from the University of Maryland School of Medicine (UMSOM) exposed groups of mice to air polluted with particulate matter.

Some of those mice were bred without the brain inflammation marker inhibitor kappa B kinase 2 (IKK2), while others were bred with the marker.

Polluted air didn’t lower sperm counts in mice lacking IKK2, while it did lower counts in mice with the marker, supporting a direct connection between brain inflammation and male fertility.

The team delved deeper, seeking to learn if the hypothalamus alone was responsible for the link between air pollution, inflammation, and sperm count. They were interested in the hypothalamus because of its involvement in the hypothalamic–pituitary–gonadal axis, which regulates sperm production.

The researchers found that overexpression of IKK2 in one of the main types of neurons in the hypothalamus, proopiomelanocortin (POMC) neurons, was enough by itself to lower sperm count.

On the other hand, when IKK2 was deficient in those POMC neurons, sperm counts in the mice weren’t affected.

“Looking back, it makes perfect sense that the neurons in the hypothalamus are the culprits perpetuating this inflammation response that results in low sperm count, as we know that the hypothalamus is a major pathway link between the brain and the reproductive system,” said Dr. Zhekang Ying, an assistant professor of medicine at UMSOM and lead author of the study.

The research adds to the growing literature linking air pollution with impaired fertility in both animals and humans.

Sperm counts are rapidly declining across the planet, with researcher Shanna Swan suggesting that sperm counts could reach zero by 2045.

The problem is particularly pronounced in developed Western nations, including the United States, Europe, Australia, and New Zealand.

Some scientists have pointed to endocrine disruptors, such as phthalates in plastics and glyphosate in the pesticide Roundup, as key culprits in the decrease. Obesity and diet also have been blamed.

Dr. Charles A. Hong, the Dr. Melvin Sharoky professor in medicine and director of cardiology research at UMSOM, said the findings have value beyond their relevance to understanding why sperm counts are in decline.

“There are many conditions, such as high blood pressure, diabetes, and heart disease, that can result from brain inflammation due to air pollution,” he said.