Atmospheric Pollution and Pre-term Birth Rate in the Los Angeles County Area


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Premature birth may be caused by atmospheric pollution in some cases. Photo by doc_

A study performed in the University of California, Los Angeles, showed an association between the pre-term birth rate and the concentration of some pollutants in the atmosphere. Poly Aromatic Hydrocarbons (PAHs) and particulates of ammonium nitrate were the chemicals with the most pronounced effect.

Combustion-related Atmospheric Pollution

The emissions of toxic chemicals, causing air pollution, may come from several sources. Three of the major sources of air pollution are power generation, domestic households, and vehicles. In all cases, the majority of the emitted pollutants are by-products resulting from a combustion process.

Combustion is the reaction of a fuel with oxygen (O2), to generate energy. Fuels (gas, oil or coal) are normally hydrocarbons; molecules with a general formula CxHy. When these molecules are burnt, they form carbon dioxide (CO2) and water (H2O). Combustion by-products, however, can be emitted when:

  • The combustion is incomplete.
  • Some impurities are present in the fuel.

Incomplete Combustion

The most common molecule caused by incomplete combustion is carbon monoxide (CO). This molecule is formed when not enough oxygen is present for the proper combustion to take place. This can occur in the engine of a vehicle or inside a power plant, particularly if the system is not new, and when it does not work properly. CO can cause a reduction in the oxygen level in the human body; if its concentration is particularly high, it can be lethal.

Other emitted pollutants due to incomplete combustion can be molecules present in the fuel which do not react with oxygen, or react just partially. This will lead to the emission of the original hydrocarbons present in the fuel, or of a molecule derived from them. For instance, a big molecule can be broken into smaller ones. Polycyclic Aromatic Hydrocarbons (PAH) are a class of compounds often emitted by vehicles as combustion by-products. They can be very toxic; for some of them, a carcinogenic effect has been proven.

Impurities in the fuel

The most common impurities present in fuels are normally nitrogen (N) and sulfur (S). Both nitrogen and sulfur can both react with oxygen to form the corresponding oxides: NO, NO2, NOx (1 < x < 2) for nitrogen, SO2 and SO3 for sulfur. These oxides can then react with water vapor, present either in the fuel or in the atmosphere, to form the corresponding acids HNO3 and H2SO4.

The presence of high concentrations of these acids in the air can result in problems for human health, causing irritations and respiratory problems. Furthermore, the environment (vegetation and animal species) can also be affected.

When ammonia (NH3) is also present in the atmosphere, it can react with both of the acids, to form ammonium nitrate and sulfate respectively.

Particulate Matter can be emitted in the atmosphere during combustion. Image by Thoursie

Particulate Matter

Particulate Matter (PM) is another combustion by-product pollutant. It is a mixture of small solid particle and liquid droplets; they can be caused either by incomplete combustion or by the presence of impurities in the fuels.

The smoke emitted for the combustions of coal or diesel fuels, for instance, contains a high quantity of PM. Some of the pollutants mentioned above, such as PAHs, ammonium nitrate or sulfate, can be present in the atmosphere in form of PM.

The danger of PM is associated not just with its nature, but also with its dimensions. This is because the smaller the particles are, the easier they can penetrate into the human body. Normally particles with diameters smaller than 10 and 2.5 mm are regularly monitored; they are referred to as PM10 and PM2.5 respectively.

Study of the Effect on Pre-Term Birth Rate

Some of the risks associated with these pollutants are well known and documented. Other effects on human health, however, are less clear, and further investigation is needed to understand them better.

Recently some data were published about the presence of some these pollutants in the atmosphere, and their possible effects on the pre-term birth rate. This study was performed by Dr. Wilhelm and her coworkers, as collaboration between the University of California, Los Angeles, University of California, Berkley, and University of Southern California.

Los Angeles smog linked to preterm birth: Photo of the LA skyline Al Pavangkanan

The researchers considered the births for a period of about two years, between June 2004 and March 2006 in Los Angeles County (California); they tried to associate the pre-term birth rate with the exposure of the women to the pollutants during their pregnancies. Several contaminants were monitored for this study, such as PAH, nitrogen oxides and PM2.5 of particular species (ammonium nitrate, particulate from biomass burning or gasoline burning, etc).


The study performed by Dr. Wilhelm and her coworkers was quite complex, as they took many parameters into account.

Features like the age and ethnic background of the women, for instance, were considered. Furthermore, the pollutants present in the air could vary, depending on the areas where the women lived – costal regions were very different from non-costal regions.

Despite this, an association between the concentrations of some of the pollutants and the pre-term birth rate was established. The species with the greatest effect were PAHs, as a 30% increase in the pre-term births was observed. Elemental carbon and PM2.5 of ammonium nitrate also showed an effect, although smaller (6-21%). These data could lead to further studies and analyses in the future, especially to understand better the effect of the PAHs.

The study was performed using data from the Multiple Air Toxics Exposure Study (MATESIII), combined with statistical modelling analysis. The complete work from Dr. Wilhelm and her coworkers is published in Environmental Health.


M.B. Hocking: “Handbook of chemical technology and pollution control.” Academic Press, 1998.

M. Wilhelm, J.K. Ghosh, J. Su, M. Cockburn, M. Jerrett, B. Ritz: “Traffic-related air toxics and preterm birth: a population-based case-control study in Los Angeles County, California.” Environmental Health, (2011). doi:10.1186/1476-069X-10-89.

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