A recently published peer-reviewed study (Woodruff et al. 1997) found a statistically significant relationship between particulate air pollution in the United States and postneonatal infant mortality. Postneonatal mortality was defined as infant death that occurred between the age of 28 to 364 days. The study analyzed the relationship between PM10 levels and post- neonatal mortality within a population of approximately 4 million infants born in 86 metropolitan areas in the United States between 1989 and 1991 (Woodruff et al. 1997).
Based on the risk factors derived by Woodruff and colleagues (1997), the Environmental Working Group and Physicians for Social Responsibility estimated the number of SIDS cases each year associated with airborne particle pollution (PM10).We estimate that about 500 SIDS cases each year in the United States are associated with airborne particle pollution.
Within the next two weeks, the EPA will finalize new regulations that will cut air pollution levels in half over the next ten years, including the particle air pollution associated with SIDS in the Woodruff study. EPA estimates that these microscopic airborne particles (PM10) penetrate deep into the lungs and cause 35,000 premature deaths in the United States each year. Influential members of Congress have threatened to overturn the new regulations, and polluting industries are waging an all-out fight to block the new health standards.
The relationship between SIDS cases and PM10 was used to predict SIDS mortality in metropolitan areas in the United States. Nearly one out of every five SIDS cases in the top twelve metro areas are associated with particle air pollution (PM10) (Table 1). The greater Los Angeles, New York, and Chicago metropolitan areas lead the nation in SIDS cases linked to airborne particle pollution with an estimated 44, 28 and 27 SIDS cases associated with microscopic airborne particles, so-called PM10, each year.
Airborne particle pollution (PM10) in just ten states is associated with more than 300 of the estimated 500 SIDS cases linked nationwide to PM10 each year. California tops the list with an estimated 93 SIDS cases associated with airborne toxic particles, followed by Texas and Illinois, with 37 and 32, respectively.
SIDS and the Woodruff Study
SIDS is defined as the “sudden death of an infant under one year of age which remains unexplained after a thorough case investigation, including performance of a complete autopsy, examination of the death scene, and review of the clinical history” (Willinger et al. 1991).
While the cause of Sudden Infant Death Syndrome is still unknown, a triple-risk model is often used to describe the confluence of events that may lead to the sudden death of an infant (SIDS Alliance 1997). According to this model, all three elements must come together for SIDS to result.
The first element of the model, the critical development period, encompasses rapid growth phases during the infant’s first six months of life which may periodically destabilize the infant’s system. During this developmental period, changes occur in homeostatic controls, such as sleeping and waking, breathing, heart rate, blood pressure and temperature.
The second element, the vulnerable infant, represents an infant with an underlying defect or abnormality. In this model, normal babies do not die of SIDS; instead, there are pathophysiological reasons behind these seemingly sudden deaths, such as defects in regions of the brain that control respiration and heart rate during early life. Most vulnerable babies, however, appear perfectly healthy.
The third element involves exogenous stressors, outside or environmental challenges which a normal baby can overcome and survive, that an already vulnerable baby might not. Stressors such as second-hand exposure to tobacco smoke, prone sleep position, an upper respiratory infection, or air pollution (PM10) alone do not “cause” death for the infant, but can tip the balance against an infant’s chances of survival (Sidebar).
After controlling for maternal smoking, poverty, age of the mother at birth, temperature, and other potentially confounding factors, Woodruff and coworkers (1997) found a statistically significant relationship between PM10 levels and three categories of infant mortality: overall postneonatal infant mortality, sudden infant death syndrome (SIDS) and infant respiratory related death. The link was the strongest for normal birth weight babies. Notably, no relationship was found for PM10 levels and infant death from other causes, which in effect provides a control for the study. Peer reviewed studies in the Czech Republic, Taiwan, and Brazil also have found a statistically significant relationship between particulate air pollution and infant mortality (Bobak 1992, Knobel 1995, Penna 1991).
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