Is wood smoke the bad guy? When particulate matter is being measured, size does matter.

Our respiratory systems have defenses that prevent airborne particles from getting into our lungs. The particles have to get by the nasal passages, trachea and conducting airways.Particles 100 micrometers and bigger are typically not drawn into the body because of their size. Particles in the range of 10 to 100 micrometers have trouble making the turns in the passages and airways.It's the particles smaller than 10 micrometers that are generally able to travel into the pulmonary part of the lungs. For example, pollen can be as small as 10 micrometers.Smog, secondhand tobacco smoke and soot typically measure 0.1 micrometers. Things that measure at 1 micrometer, which can be inhaled into the lungs include cat allergens, viruses, bacteria, milled flour, metal fumes, clay silt and house dust mites, according to the National Institute of Environmental Health Sciences (www.ehponline.org)Opacity regulationsMeasuring opacity levels is complicated because of all the factors that could be involved, such as visual angles, the type of steam plume, mathematical calculations regarding distance and height of chimney, and weather conditions. The standard for time of measurement is six minutes.One nationally recognized training program is run by the Eastern Technical Association. The training manual is 39 pages. (www.smokeschool.com).