In this and the next part, we will look at health impacts associated with two specific pollutants, aerosol particles and radon gas. This part is about the particles.

1

2

Let’s first look at a few examples of the materials we find in aerosol particles. <> Asbestos is a mineral that shreds into fine fibers instead of somewhat-spherical particles like other rocks. The fact that minerals withstand extremely high temperatures without burning or melting allowed asbestos to be incorporated into heat insulation materials and stuff that benefitted from the fibrous structure and heat resistance, such as floor and ceiling tile and brake linings. The problem occurs when the asbestos-containing material deteriorates and the asbestos fibers become airborne. The fibers, when breathed in, become stuck inside the respiratory system and cause chronic inflammation. The particles become encapsulated by scar tissue and protein-like coatings. Asbestosis results from the chronic inflammation; lung cancer, such as mesothelioma, may result from tumor growth induced by the encapsulation of the particles or the carcinogenic quality of the asbestos. <> Another material sometimes found in particles is lead. In the old days before unleaded gasoline, there were measurable amounts of lead particles in the outdoor air. Nowadays, it’s mostly indoors from the erosion of old house paint that used lead-based pigments. Industrial operations where lead is smelted or melted down can release lead particles into the air. Inhalation or ingestion of lead can lead to neurological problems, mental illness, anemia, and other effects. Actually, not a lot of lead is inhaled; most of the lead that enters the body is through ingestion, but the lead may have spent time in the air as particles and settled out in food-production areas and became incorporated into the foods. <> There are other toxic and carcinogenic metals that occur in particles, including chromium, mercury, arsenic, nickel, and so on.

3

Here is a photomicrograph of asbestos particles on the right. The fibers are probably less than a micrometer in width, and up to twenty or thirty micrometers long. These particles are smaller than a human hair, which may be a several tens of micrometers in diameter. The x-ray photograph on the left shows shadows in the lungs, which are areas of asbestosis.

4

Other particulates may originate as smoke from combustion. The visible part of smoke are particulates; the invisible part consists of various combustion gas molecules. One type of particle formation involves the clumping of gas molecules until they become big enough to scatter light and become visible. This is known as gas-to-particle conversion, and can occur with pollutant gases in smog and among the gases in combustion exhaust. These make up some of the smallest aerosol particles, and the light they scatter is what makes the air “hazy”.

5

Here is some of that haze. Some of the particles in this haze may be small soil dust particles and some may be very fine water droplets.

6

Particulates are very small, perhaps with radii as small as a few hundredths of a micrometer up to ten or twenty micrometers. This chart shows, besides the general sizes of various types of particles, the typical fallspeeds of the particles. This is the average speed that particles fall toward the ground due to gravitational force downward against the aerodynamic friction of air flowing around the particles as they fall. Generally, the smaller the particle, the slower it falls and therefore the longer it stays in the air. This means that we do not find many particles in the air that are larger than about ten to twenty micrometers radius, because they fall so fast, they do not stay in the air for very long. It also means the smallest particles are potentially the most dangerous, because they stay around in the atmosphere long enough for someone to breathe them in.

7

The reason we have this specific lecture section on aerosol particles is that studies over the years have shown that exposure to particulates has a particularly strong impact on health, especially the “fine” particles that are smaller than two to three micrometers diameter. This plot shows how the mortality rates are fairly linearly correlated to the concentration of the fine particulates.

8

This is an example of a news article about studies showing the correlation of particulate exposure and health risk. Children are especially susceptible to the health impacts from all kinds of air pollutants, not just particles, because small children breathe in more air per unit bodyweight than adults.

9

We can classify the health impacts of particles, and other pollutants for that matter, as follows: <> Toxins are basically poisons, where they have a harmful effect on physiological functions, like breathing, central nervous system functions, liver and kidney functions, etc. <> Carcinogens, however, involve mutations and non-differentiated growth of tissue in the form of tumors. The term “mutagen” may apply here, due to the mutations of DNA, but not all mutations result in cancerous growth or a risk of cancerous growth, so we should reserve the term “carcinogen” for those. <> A “promutagen” is something that indirectly causes mutations, by helping to form other mutagenic agents in the body. Some promutagens may therefore be considered a type of carcinogen.

10

To determine if a substance is a carcinogen, we need to see if it will increase the risk of tumor growth. Doing this in a conventional toxicological study takes too long, since it usually involves the entire lifetime of the subject. So we need something that has a shorter lifetime or faster life process, like bacteria. This is relatively inexpensive as well, since we can analyze thousands of bacteriophages for mutations.

The AMES test involves exposure of Salmonella bacteria to the test mutagen. The initial strain of Salmonella requires histidine to flourish. After exposure to the mutagen, the bacteria are cultured on a histidine-poor medium. Mutated Salmonella lose their need for histidine and grow while only a few colonies might survive in the unexposed control group due to a random mutation. The greater the bacterial growth in the exposed group, the greater the carcinogenic effect of the pollutant, assuming the mutation effect leads to increased cancer risk. And now, a few questions about particles and health.

11