With the aid of an imaging technique that uses a radioactive isotope, researchers in Belgium have shown that small particles, similar in size to those found in air pollution, can enter a person's bloodstream from the lungs.

According to the investigators, the findings could help explain the ``poorly understood'' relationship between air pollution and cardiovascular disease--such as the recent evidence tying pollution exposure to heart attack risk.

These new findings ``came somewhat as a surprise because it is generally believed that the lung-blood barrier is very impermeable for particles and only allows the passage of gases or vapors,'' Dr. Ben Nemery, of the Katholieke Universiteit Leuven, said in an interview with Reuters Health.

``So, what we showed is that this barrier may be more leaky than we thought, at least for the smallest particles,'' he explained.

Small particles are abundant in urban air since they are produced by diesel-powered engines--and are possibly the ones that are the most harmful to health, Nemery noted.

``The fact that they can penetrate into the blood circulation could explain why people may die from cardiovascular causes--like heart attacks--as a result of increased urban air pollution,'' he said. ``However, there are also other possible mechanisms, and we do not claim that our data gives a final explanation.''

In their investigation, the researchers tracked the movement of radioactive particles after five nonsmoking men between the ages of 24 and 47 inhaled them. Immediately after inhalation, the men underwent body scans that monitored the particles as they traveled from the lungs, into the bloodstream and to different organs including the liver and bladder. The men also had several blood samples drawn throughout the scanning procedure, which lasted about 60 minutes.

``The radioactivity was detected in blood already after 1 minute, reached a maximum between 10 and 20 minutes, and remained at this level up to 60 minutes,'' the authors report in the January 29th issue of Circulation: Journal of the American Heart Association (news - web sites).

They note that the radioactive ultra-fine particles used in the study, and considered to be ``very similar to the ultra-fine fraction of actual pollutant particles,'' travelled rapidly into the bloodstream.

The finding ``should be considered relevant for the cardiovascular (illness and death) related to ambient particle pollution,'' the investigators conclude.

``Small particles stay airborne for a long time,'' Nemery told Reuters Health. ``These particles may be harmful by themselves or because they carry toxic--or carcinogenic-substances on their surface.''

However, he added, ``the danger of inhaling particles is mainly for people who are already ill with heart or lung disease.''