Boon Lim1,2, Associate Professor
1. Department of Obstetrics and Gynaecology, Canberra Health Services,
Canberra, Australia
2. Australian National University
The United Nations Environmental Programme (UNEP 2022) has declared
wildfires more numerous & intense than ever before. The effect of
climate change on the rate of bushfires is intuitive – rising earth
temperatures & extreme weather combine dry vegetation with stronger
low-humidity winds to create a perfect fire storm. Eastern Australia &
the West Coast of the United States of America regularly experience
summer fires but the 2019-2020 fire seasons saw record-breaking numbers
and extent of wildfires with devastating consequences.
Exposure to wildfire smoke during pregnancy has been associated with
increased maternal stress, higher rates of gestational diabetes &
hypertension, preterm birth & birthweight reduction. A systematic
review regarding wildfire exposure in pregnancy & adverse birth
outcomes determined higher incidence of preterm birth due to wildfire
exposure particularly during the second trimester. The release of
PM2.5 (particulate matter smaller than 2.5 μm in
aerometric diameter) by wildfire is a known risk factor for preterm
birth. Ambient PM2.5 for entire gestation has been shown
to induce birthweight reduction (Amjad S. Environ Int.,2021:156:106644).
Data from the Canberra, Australia fires of 2003 showed that male infants
born in the most severely fire-affected area had significantly higher
average birth weights than their less exposed peers and were also
heavier than males born in the same areas in non-fire years (O’Donnell
M. Evolution, Medicine, and Public Health 2015:344–354)
Historically, environmental climate conditions have been shown to alter
sex ratio in human births. Sex ratio (the ratio of males to females in a
population), can be observed at conception (primary sex ratio, PSR) or
at birth (secondary sex ratio, SSR). Lyster first described a
significant reduction in SSR (109 compared to 144), approximately 320
days following the London smog from 5th to
9th December in 1952, in an otherwise masculine sex
ratio period (733 vs 623) (J Obstet Gynaecol Br Commonw.,
1965:81:626-631). Male frailty, maternal stress, air pollution and
ambient PM exposure, & radiation exposure have all been implicated as
causes.
In 1984, O. Lloyd suggested “changes in the sex ratios … may be
an early indication of changes of general mortality or morbidity
resulting from chemical or biological stresses in the environment”
after discovering the highest sex ratio of births in Armidale, Scotland
in 1967 coincided with an acute increase in mortality from non-malignant
respiratory disease, respiratory cancer & non-respiratory cancer. They
hypothesised this was due to air pollution, namely high concentrations
of airborne carcinogenic ‘trigger factors’ from local foundries (BJOG,
1984: 91:901-907) (Fig. 1). Interestingly, only one natural disaster has
been associated with a subsequent rise in sex ratio following it -
Hurricane Katrina in 2005 (Med Princ Pract. 2015. 24:5: 477–485).
The data for wildfires is limited but consistent. O’Donnell’s analysis
of the 2009 Black Saturday bushfires in Victoria, Australia found a
significant decrease in the secondary sex ratio of births conceived
after the fires, with a male birth rate of 46.6% in the severely
affected regions, in comparison with a male birth rate of 51.1% in the
remainder of Victoria (O’Donnell, 2017. p172-173).
With natural disasters becoming reportedly more frequent and more
extensive, the impact on birth outcomes needs to be better researched to
assist with planning for health care in order to reduce the risks of
perinatal morbidity and mortality.
Figure 1.