Smog and Particulates

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Smog and Particulates

How Fine Particulates Affect Exterior and Interior Air Quality

No matter how well our “Green and Healthy” homes are designed and built, our interior air quality can only be as good as our exterior air quality (our “source air”), unless we add active air filtration into our building design.  But how do we determine what type of filtration and what size system is necessary?  A larger than needed system is wasting energy and money to purchase and install.

Most air filters remove particulates, while very few filters remove gasses such as VOC’s, Carbon Monoxide, and other chemical gasses dissolved in the air we breathe.

Particulates are defined into two main classes by the US EPA as:

“Inhalable coarse particles,” such as those found near roadways and dusty industries, are larger than 2.5 micrometers and smaller than 10 micrometers in diameter.”  &

“Fine particles,” such as those found in smoke and haze, are 2.5 micrometers in diameter and smaller. These particles can be directly emitted from sources such as forest fires, or they can form when gases emitted from power plants, industries and automobiles react in the air.

Mold spores, pollen, bacteria, and other biological respiratory contaminants can fall under either Coarse or Fine particles depending on size.

Inhalable coarse particles can be irritating to the nose and throat, but are usually captured by the mucous lining the nose, throat and trachea, and are fairly easily expelled from the body.

Fine particles, on the other hand, usually migrate into the lungs and can become lodged in the alveoli and can be very difficult for the body to expel.

From a health standpoint, fine particles are a greater health concern.

The US EPA has set National Ambient Air Quality Standards for many air pollutants including smog and particulates.   The PM10 Standard sets the limit for Inhalable Coarse Particles (Less than 10 micrometers in diameter) at 150 micrograms/cubic meter for a 24 hour average, not to be exceeded more than once per year on average over three years.

The PM2.5 Standard, set for fine particles, is a bit more complicated, with the annual average limit being 12 mg/m3 as a primary standard and 15 mg/m3 as a secondary annual average standard.  The 24-hour average limit is 35 mg/m3.

You can determine the level of  smog and particulates in the air in a few ways.  Larger municipalities often have a local Air Quality Management District.  The area where I live, the San Francisco Bay Area, has the Bay Area Air Quality Management District.  The BAAQMD samples air in multiple locations and measures against EPA NAAQS.  The BAAQMD web site publishes Air Quality Summaries and you can look at the reports and examine the air quality report for the sampling location closest to where you live.

Particle Counter

Particle Counter

You can also directly measure the particulate level both inside and outside your home, or any other location, by using a laser particle counter.  I use a Lighthouse Brand handheld particle counter that can calculate both PM10 and PM2.5 for comparison to the EPA standard.  One sampling data set cannot provide any useful information.  Multiple samples and data logging over multiple days are necessary to get a good idea of the actual particulate levels, as variables such as time of day, wind speed and direction, weather and local barometric pressure, and traffic conditions can all affect measurements.  Usually I set up the particle counter to take a 2-minute sample every 15 minutes for at least 48 hours (I prefer 120 hours, or 5 days).  The data from this sampling can then be graphed and analyzed to get a good picture of the average particulate levels, as well as peaks when particulates are especially high (such as rush hour near a freeway).

The information from a particle counter survey can be used to determine the best and most appropriate filter system to install to most effectively remove both coarse and file particles.  Best practices can also be implemented to reduce exposure.  If there are significant peaks in fine particles you may choose to close your windows and not have your children play outside during the peak times, or pick times for outdoor activities when exterior particle counts are lowest.

Most household dust is fairly large in size and would be in the coarse particle category, or greater that 2.5 mm in diameter. Thus most of the fine particles found in interior air migrate in from the exterior.  True HEPA filters remove 99.97% of particles down to 0.3 mm in diameter.  A true HEPA filter can help reduce the amount of particulates in the interior air.  Most residential HVAC filters do remove particulates, but often do a poor job removing the smaller, fine particulates.

Conducting a particle count survey can provide very useful information regarding the air quality around your home, and can help in choosing strategies to reduce exposure to these harmful contaminants.