This week’s historic red alert for air pollution in Beijing prompted me to research and write an article on another time in recent years when the world’s attention was focused on the Chinese city. In 2008, the eyes of the world were on Beijing during the Olympic Games between August 8 and September 24. An article published in Environmental Health Perspectives in September this year investigated whether birth weights in the time period of the 2008 Olympics were higher than for August 8/September 24 2007 or 2009, and reported some interesting results.
The research for the article was a collaboration between Universities in the United States including The University of Rochester, Rutgers University and Duke, and Universities in China including the Capital Medical University and the Chinese Research Academy of Environmental Health Sciences (both in Beijing). The researchers investigated more than 83000 full term births to mothers in four urban districts of Beijing (Xicheng, Haidan, Fengtai and Chaoyang) and in addition to estimating the difference in birth weight between births during the Olympics and the same time period in 2007 and 2009, they also estimated the difference in birth weight associated with the interquartile range increases in concentrations of PM2.5, Sulphur dioxide (SO2), Nitrogen dioxide (NO2) and Carbon monoxide (CO) during each month.
The article discussed a number of previous studies which examined the association between exposure to air pollution at certain stages of pregnancy... and birth weight
The article discussed a number of previous studies which examined the association between exposure to air pollution at certain stages of pregnancy (e.g. first, second and third trimester or entire pregnancy) and birth weight – however inconsistent findings in these studies were linked to differences in study design, data sources, pollution exposure errors (due to data from monitors or land use regression estimates) and confounding by specific characteristics of individual subjects. The researchers also discussed data from one of their previous studies which focused on a 13 month closure of a large steel mill in the Utah Valley (United States) when the PM10 concentrations were lowered by more than 40%. The results of the US study showed that mothers who were already pregnant at the time of this closure were less likely to deliver prematurely that mothers who were pregnant prior to or following the closure. However, the results did not show a significant reduction in birth weight for those babies born during the closure.
During the period of the 2008 Olympics, the Chinese Government implemented a range of measures as part of an agreement to improve air quality in Beijing. These measures included implementing stricter vehicle emission standards, restricting the use of vehicles on the basis of the licence plate number, closing and relocating industrial facilities in Beijing and surrounding province, and stopping construction activities. It was reported that the concentrations of PM2.5, NO2, SO2, and CO (amongst others) decreased by 18-59% during the time of the Olympics compared to concentrations prior to the event. While there was an overall decrease in pollutant concentrations, variations in meteorological conditions resulted in large day to day variations before, during and after the Olympics.
Birth data was taken from the Beijing Obstetrics and Gynaecology Hospital register. All infants that had at least 28 weeks of gestation between 1 January 2007 and 31 December 2010, and were born to mothers who resided in the four adjacent Beijing districts. Infants with birth weights greater or less than 5 standard deviations from the mean and those with more than 41 weeks of gestation were excluded. Daily concentrations of PM10, NO2, SO2 averaged across the pollution monitoring stations in each of the four districts from 8 August to 24 September in 2007, 2008 and 2009 were collected. The research team also used previously collected pollution and weather measurements which included PM2.5 and CO.
The results of the study showed that if the eighth month of pregnancy coincided with the 2008 Olympics, the mean birth weight of infants born at full term was 23 grams higher than for corresponding pregnancies in 2007 or 2009. This was independent of factors including the mothers age and education, the mothers residential district, gestational age, and pregnancy complications. Interestingly, the difference was found to be greatest among mothers with a minimum of a Bachelor’s degree.
In discussing the strengths and limitations of their study, the researchers did not rule out potential confounding by socioeconomic status. If women of lower socioeconomic status (e.g. lower education levels) who are at higher risk of adverse pregnancy outcomes moved away from the study regions during the Olympics, the researchers speculated that this may explain the estimated increase in birth weights. On further examination of the results, however, this was not found to be the case. They also pointed out that a 47 day reduction in air pollution concentrations may have been too short to examine the beneficial reproductive health effects if the improvements occurred early in the pregnancy. Although the increase in weight during the Olympics relative to the median birth weight of the infants was very small (less than 1%), the researchers speculated that the increase may have been larger if the reduced air pollutant concentrations lasted for several months or for the entire pregnancy.