Today’s episode is about a study published in January that looked at how outdoor air pollution impacts the sex of babies conceived through in-vitro fertilization (aka IVF). 

Study Setup

• There were 7,004 participants- this high number of participants is good because it helps to make the results more generalizable and less likely to be biased towards any sort of extreme.

• Participants were babies born from fresh or frozen SINGLE embryo transfers between 2013 - 2017. This brings me to the second thing I really liked about this study - these embryo transfers took place relatively recently, which also helps to make the results more relatable to current scenarios with IVF. In past episodes, like episode #6, I mentioned that a potential weakness of some IVF studies is including babies born in different eras of reproductive technology, since we have fortunately advanced so far with IVF since the 1990s and even early 2000s.

• Embryos were included:

  • transferred as singles

    • they could not have been born from a multiple pregnancy, because this could affect the embryo’s development

  • transferred from in vitro fertilization IVF or intracytoplasmic sperm injection (ICSI)

• If the embryos received pre-implantation genetic testing before transfer, then they were excluded. I appreciate that the study kept tabs on a lot of factors that might impact the embryo. They noted the age of the embryo’s parents, if the father was a smoker, which types of stimulation medications the mother used to prepare for IVF, the quality of the embryo, and whether the embryo was a fresh or frozen transfer. Any of these factors could have impacted the results and altered the conclusions, and one of those factors did come into play…

Now that you know who was included in the study, it’s worth mentioning where the study occurred, which was in China in a place called Xiamen in the Fujian province. Xiamen looks like an attractive seaside city - they have a university there to train Buddhist monks, and the air quality is better there as compared to many other parts of China. It ranks in the top five major cities for good air quality in in China. This relatively cleaner air was critical to my interest in the study, because realistically, studies that mirror conditions similar to those found locally draw stronger parallels. Air quality data from three neighboring monitoring stations was used to approximate the pollutant concentrations inside of this IVF center. At the time of recording, the air stations closest to the IVF site in Xiamen are reporting worse air quality than New York City but better air quality than Los Angeles.

Looking at the air quality data from the three nearby stations, the study measured pollutant concentrations of carbon monoxide, ozone, nitrogen dioxide, and sulfur dioxide in relation to the ratios of males to females born after embryo transfer. Of those pollutants, the only one found to have an association not likely due to chance was sulfur dioxide (SO2). 

SO2 gets into our air from the burning of sulfur-containing fossil fuels in industrial facilities and power plants. Chronic exposure to SO2 can negatively impact the lungs, though people with asthma may be triggered by quick exposure to low levels. It is colorless and as a distinct smell like rotten eggs.

With higher levels of SO2, they found decreased implantation and higher rates of miscarriages, regardless of the quality of the embryo. 

At lower levels of SO2, there were twice the number of males to females born from embryos of moderate to lower quality. (They did not see a difference in ratio in the top quality embryos). After accounting for all of the variables mentioned earlier, like if the father had been a smoker or whether the embryo came from a fresh or frozen transfer, this doubling of males to females embryos still held true.

With these results, the study authors hypothesize that there are differences in how male and female embryos develop and that there are differences in how male and female embryos respond to the stress of exposure to SO2. This makes sense in the context of a 2015 study that found that air pollutants impact the sex ratio of children born from spontaneous conception.

Since the results have just led to more hypotheses, more research is certainly needed. The authors brought up how their IVF laboratory uses HEPA filters to reduce undesirable chemicals floating in the air. (Just an FYI, HEPA stands for “High Efficiency Particulate Air,” and they are often used in U.S. healthcare sites as well). Though HEPA filters remove nitrogenous oxides, they don’t reduce SO2. Thus, some questions are, should HEPA filters be re-designed to better filter out SO2? Also, if pollutants are still in the air in an IVF lab, how much do they stick to the culture medium growing the embryos?

Hopefully we will have the answers to questions like this in the future. For now, the main thing I recommend for women trying to conceive is to minimize time spent in areas with poor air quality, because at best SO2 will steer you towards having a boy, and at worst, it could increase the chances of miscarriage. Also, I’d recommend knowing more about the air quality where your fertility clinic is located. Given that the deaths of several children have been associated with poor air quality in the operating rooms at Seattle Children’s Hospital, it’s worth doing your own due diligence to inquire about the air filtering systems used in the embryo lab where your own children will be growing.