Negative health effects from the chronic inhalation of polluted air are well known to cause cardio-respiratory disease. It can be particularly damaging to seniors, children, and people with asthma.

Now according to a study from Ohio State University, breathing polluted air can also cause widespread inflammation by triggering the release of white blood cells from bone marrow into the blood stream. The influx of white blood cells can alter the integrity of the blood vessels. The white blood cells are then absorbed into fat tissues where chemicals are released that cause inflammation.

The main culprit in the air that causes this to occur is fine particulate matter, or PM. How exactly the PM triggers this cascade of cellular activity remains unclear. Scientists hypothesize that the damage may originate in the fluid lining of the lungs. When molecules in this fluid are exposed to PM, their structures change, activating the toll-like receptor 4 (TLR4).

TLR4 recognizes the specific characteristics of pathogens and activates the immune system, the white blood cells in particular.

"Our main hypothesis is that particulate matter stimulates inflammation in the lung, and products of that inflammation spill over into the body's circulation, traveling to fat tissue to promote inflammation and causing vascular dysfunction," said Sanjay Rajagopalan, professor of cardiovascular medicine at Ohio State and senior author of the study. "We haven't identified the entire mechanism, but we have evidence now that activation of TLR4 influences this response."

To test this theory, the researchers used groups of mice, exposing them to clean air and air with 8-10 times more fine particulate matter than ambient urban air (11ug/m3). Their exposure lasted six hours per day, five days per week, for twenty weeks. The effects were compared to normal mice and mice deficient in TLR4.

For normal mice, the group exposed to polluted air showed higher levels of white blood cells in their spleens and blood. The white blood cells found are known as inflammatory monocytes. For mice deficient in TLR4, the effect on white blood cells was lessened. This suggests that a lack of TLR4 prohibits the release of inflammatory monocytes.

Another compound was found to increase with greater polluted air inhalation, superoxides. These are meant to attack pathogens in the body, but can be toxic if there are none to attack. They produce an enzyme that causes inflammation, NADPH oxidase. Particulate matter is mistaken in the body as a pathogen, such as a virus. However, it is nothing of the sort. Triggering the release of elements in the immune system when there is no pathogen can do more harm than good.

The Ohio State University study has been published in the journal, Circulation Research.