“Nearly 40% of preterm births are attributable to air pollution.”
Around 15 million babies are born prematurely every year worldwide. Premature birth brings immediate health challenges to the newborn determined by how early the baby is born. There are also long-term impacts due to the development of a wide range of chronic respiratory, cardiovascular, neurological, and other disabilities. These long term effects of preterm birth cause around 13.2 million years lived with disability which is a measure of the number of years with a lower quality of life due to disease. Preterm birth is also the leading cause of death in children under the age of 5 years.
Despite many advances in the care of pregnant women to limit the likelihood of preterm birth, rates of preterm birth continue to increase in many countries with the greatest burden in low-middle income countries. Nearly 40% of preterm births are attributable to air pollution, especially ambient and household PM2.5. Air pollution exposures of pregnant women have also been shown to be associated with low birth weight which also brings immediate and longer-term health impacts for the child.
In a UK study of babies born within the M25 perimeter over a 5-year period, some 3% of low birth weight babies born at full term was attributable to increasing air pollution exposures, especially PM2.5 from traffic. Other negative effects of air pollution on pregnancy outcomes are emerging and include pregnancy loss. Fetal development during pregnancy can also be affected and this is a critical window of susceptibility that can alter the development of tissues and organs leading to detrimental health outcomes at birth and in later life. This is known as fetal programming and underpins the concept that events in early human development determine many health outcomes as we get older – the developmental origins of health and disease.
Much of our understanding about the relationship between air pollution exposures in pregnancy and health outcomes at birth and in childhood and beyond come from epidemiological studies and animal models. However, efforts are now being made to understand how these air pollution exposures cause effects on the baby’s growth and development.
The placenta is the gatekeeper between mother and fetus and can be collected at birth for study. The focus of our attentions is to try and understand what effects airborne material exposures of the pregnant woman have on placental function and what this means for the development of the fetus. We know these air pollution exposures can cause changes to the expression of different genes and protein in the placenta. We can also collect a sample of blood from the umbilical cord that contains baby’s blood as it moves from the baby to the placenta and back again and see effects on gene and protein expression here as well. These changes to gene and protein expression can translate to altered function of the placenta and cells in the baby’s blood and determine the fate of many different tissues and organs as they develop.
One of the greatest challenges is to determine how maternal air pollution exposures transmit to the placenta and fetus. Broadly speaking there are two main ways this can happen. Firstly, the pregnant women could be responding to her personal air pollution exposures and sending signals from her airways that then pass through her circulation to reach and act on the placenta and the baby. Given that respiratory tract physiology changes normally with pregnancy it is feasible that these signals might be different in pregnant women. We know very little about this yet and need to think not only about how these might differ in pregnant women versus the general population but how they might also change over the course of pregnancy. Secondly, air pollutants themselves might be passing directly from mother to the placenta and the baby. There are now a few studies demonstrating the air pollution nanoparticles can be found within cells of the placenta and the vessels carrying fetal blood through the placenta. This suggests a possible direct effect of at least the particulates within air pollution on placental cells and probably their onward passage to the fetus. These direct and indirect effects of air pollutants on the placenta and various fetal cells, tissues and organs would contribute mechanistically to altered function of these by then affecting gene and protein expression. This would explain the link between maternal air pollution exposures and health outcomes of the child such as altered brain function, performance, and behaviour.
Therefore, understanding the effects of air pollution on pregnant women and the baby developing in the womb is critical if we are to alleviate the immediate and longer-term health burden of air pollution exposures in the youngest and most vulnerable members of our communities. We especially need to understand the interaction between air pollution exposures at different stages of pregnancy alongside other determinants of health outcomes for both mother and child such as obesity. Ideally, we need to reduce air pollution exposures of pregnant women.