Air pollution may enter the placenta

Air pollution has a devastating and life-long impact on children’s health. Children are especially vulnerable to poor air quality and globally more than 93% of children live in environments where the outdoor concentrations of ambient pollutants exceed the World Health Authority (WHO) air quality guidelines. Despite the fact that we have a clear understanding of the significance of air pollution to human health we have a long way to go before we gain a full understanding of the pathways and mechanisms which determine health outcomes.

Children are particularly vulnerable to poor air quality during fetal development and in the first few years of life when exposure can result in life long changes to development and permanent damage to lung tissue. Although previous studies have shown that it is possible for fetal development to be affected by poor air quality, this study is important because it is the first study to demonstrate that black carbon can cross into the placenta.

However, linking the exposure of the mother to a specific air pollutant such as black carbon, with specific health outcomes of the foetus (and children as they grow up), is very difficult. It relies on accurate estimates of exposure and a clear understanding of the biological pathways that the pollutants affect. Further, untangling the confounding effects of exposure to other pollutants and variables such as poverty, nutrition, poor housing conditions, unrelated maternal and childhood health conditions and access to health care is problematic.

Exposure to air pollution is determined by the concentrations of air pollution an individual breathes in. We describe this as a product of the ambient concentration of each microenvironment a person spends time in multiplied by the length of time spent in that environment.

In high income countries like New Zealand, urban air quality is often dominated by traffic emissions which are also an important source of black carbon. This is true for Auckland. In such cities, we know that concentrations of air pollution vary significantly in time and space and we see steep gradients in concentration near busy roads. We also know that for adults in cities like Auckland, daily exposure is primarily determined by time spent in the commuter microenvironment. However, in the specific case of black carbon the importance of exposure the indoor environment is largely unquantified and poorly understood. Sources of black carbon in the indoor environment include wood burning stoves.

Black carbon is a tricky pollutant to quantify. It is a particle, but it is typically found stuck to other particles in the atmosphere, and may exist as part of a very small particle (known as an ultrafine particle) or as a part of coarser particles such as PM2.5 of PM10.

The size of the particle determines how far into the lung it can travel, and the latest research suggests that the smallest particles have the potential to cross through the lungs into the blood stream where they can do a lot of harm.

Depending on the size of the particle we use different methods to measure exposure. For larger particles we measure them by mass, and for smaller particles we count their number. There are no regulatory requirements to measure black carbon, and there are only a few measurements available for Auckland. It is very difficult to compare measurements made for black carbon using different instrumental methods as they are all based on different assumptions.

Further, due to the spatial variability of emission sources, and changes in the way the pollution is dispersed, it is likely to be very difficult to provide a single estimate of exposure to black carbon for the population of Auckland. I therefore don’t think it is helpful to compare the concentrations that the participants of the study were exposed to with measurements made in Auckland. However, we do know that there is not a safe limit of particulate pollution below which we can be sure that the risk of poor health outcomes is minimal. It is therefore prudent to try and reduce personal exposures as much as possible to reduce the risk of poor health outcomes.

The original study from Belgium reports the ‘high’ exposures were of the order of 2 mg/m3 modelled at near-road locations in Belgium. Data from GNS shows similar concentrations at near-road locations in Auckland.

The methodology of how the Belgian exposures were calculated is barely mentioned, making it very hard to comment on.

However, based on what is provided I think it’s reasonable to assume that the exposures in the Belgian cohort would be comparable to what you would find in urban New Zealand.

There is considerable epidemiological evidence that when a pregnant mother is exposed to air pollution there are long-term consequences for the health of her offspring. The biggest risk is for low birth weight, which significantly increases life-long risk for a collection of diseases, including diabetes, cardiovascular disease, asthma and stroke.

The reason air pollution has these effects on the offspring has puzzled scientists and doctors. The current report by Bove et al sheds some light on this by showing that inhalation of black carbon particles can accumulate in the placenta. Using 20 women with otherwise normal pregnancies who breathed air of varying quality across their pregnancies, Bove and co-workers also showed that the amount of black carbon particle accumulation in the placenta appeared to be associated with the ambient black carbon particle concentration in the air they breathed.

The authors take pains to emphasise that the black carbon is present on the fetal side of the placenta, but nowhere do they show that black carbon is present in the fetus itself. Since one of the functions of the placenta is to act as a barrier preventing toxins passing from the mother into the fetus, the placenta could be seen here as performing its normal job – accumulating the black carbon particles so they don’t get into and damage the fetus. But they could be damaging the placenta; and poor placental function could explain the low birth weight that has been associated with air pollution in other studies.

Perhaps the most alarming aspect of this study is that black carbon particles from air that is not even considered to be particularly polluted by WHO standards is nevertheless accumulating in the placenta, where it could be affecting the health of the unborn child across its entire life. The prospect of disease acquisition by this route adds a whole other dimension to how we get sick; and with ever-decreasing air quality across the world, presents a potentially enormous burden on global health and disease.