Sunday, March 15, 2015

The Air Pollution We Breathe: Where does it come from?

Over the years a number of “truths” have been reasonably well established relative to air pollution with chemicals.   One of the strongest of these is that our exposures to air pollution are typically dominated by what we breathe while indoors.    There are a number of reasons for this:
  1. We spend the vast majority of our time indoors
  2. Most outdoor pollution can penetrate to the indoor environment
  3. Mixing air change rates of fresh air are invariably lower indoors than they are outside
  4. Indoor sources of chemical pollutants often dominate over outdoors sources

One way of looking at our world from the perspective of inhalation exposure is to divide the pollution we experience  into two primary sources; namely,  near-field and far-field.    The cartoon below is designed to provide some insight into this dichotomy.

We all know and many of us are concerned about large far-field sources of air pollution such as industrial emissions from smoke stakes or from other large sources like automotive emissions, large spills or releases or forest files.    SMOG or ozone alerts fit into this category.   Indeed, the majority of regulatory effort such as laws like the Clean Air Act have  been directed at these sources.   They are clearly important; however, it has been shown that indoor sources of pollution can be just as and, in some cases, more important that the large sources.  

Over 30 years ago Lance Wallace from the EPA conducted pioneering studies under the acronym:  TEAM  or Total Exposure Assessment Methodology  
He concluded that indoor airborne concentrations resulting from essentially all eleven target chemicals that he looked at were greater than the outdoor concentrations at various locations in the US.   He and his team were the first to find that in most case, even in locations with concentrated industrial sources, the indoor sources significantly outweighed the impact of traditional far-field sources such as chemical plants, petroleum refineries, local dry cleaners or service stations.
It is worth noting that this study was done by the US EPA whose primary charge is to focus on non-occupational sources and exposures.   Presumably, the vast majority of significant exposures are even more intense at work and are from near-field sources. 

How does one measure near-field inhalation exposure?   The direct method is to monitor the breathing zone of persons while the source is present or while they are engaged in activity that will generate a near-field source.   For example, applying hair-spray might be considered a fairly intense acute near-field exposure event to the chemicals present within the sprayed product.    Other relatively intense near-field sources include any activity that releases a toxicant and that is done at “arm’s length” or at “tool’s length” such as digging, mopping or sweeping.    Measuring and timing these acute exposures will allow one to reasonably characterize them.

Another form of near-field exposure in this context are the relatively constant emissions and exposures that may be coming from indoor sources like building products.   Here “capturing” the time or spatial  element of the resulting concentrations may not as critical especially if the sources are “spread-out” like off-gassing paint or carpeting.

Near-field exposures indoors can also be modeled but here the terminology can get a bit confusing.   If one considers the entire volume of the residence as the near-field then one does not have to use a “near-field” exposure model which sizes the near-field volume as the conceptual geometric space (often a sphere hemisphere or a cylinder) which includes the source and the nose and mouth of the exposed person.   If that volume includes the entire house or entire room within a house and the source is “spread-out” as mention above, then a well-mixed box model could be a good choice.   If, however, the source and the person are only in a fraction of the room's volume, then one would use the 2 zone model with the near-field including the volume containing the source and the breathing zone.  The far-field would include the rest of the room’s volume.

It is important to remember what Lance Wallace taught us; namely, that most human inhalation exposures occur in the near-field.   Of course, dermal exposure, almost by definition, occurs within the near-field.

As usual I would love to hear of your thoughts about and experience with the near-field exposure assessment especially compared to what might be happening or not happening in the far-field.

1 comment:

  1. Your post is mentioned on indoor pollution facebook!