Exposure to air pollution in the work and residential environment during pregnancy and adverse birth effects

Air pollution is an environmental health risk that causes millions of premature deaths every year around the world. Air pollution exposure has been associated with respiratory and cardiovascular morbidity and an increased risk of lung cancer, and has also been associated with adverse birth effects. There is still some uncertainty about which compounds in air pollution that causes adverse births effects. Few studies have assessed the association of occupational exposure to air pollutants and birth outcomes even though levels can be substantially higher in occupational settings than in ambient air in the residential area. No previous studies have assessed the joint effects of air pollution in residential and occupational settings in relation to adverse birth effects. The overall aim of the study was to assess the relationship between the mothers’ exposures to air pollution in the residential and occupational environment during pregnancy and negative health effects in their children, looking specifically at birthweight, small for gestational age and preterm birth.

The first two papers consisted of mother-child pairs from single births (995,843 observations) during 1994 to the end of 2012 and the occupational exposure to particles were assessed with a job exposure matrix. In the last paper we restricted the sample further, to include all single births from the beginning of 2007 to the end of 2012 (546,618 observations), and used a modelling system called SIMAIR to assess the exposure to air pollution in residential settings. All of the data in these nationwide cohorts came from Swedish national registers, which included important potential confounders and outcome variables. The results show that working mothers with low absence and high exposure to organic and inorganic particles had an increased risk of adverse birth effects in form of preterm birth, low birthweight and small for gestational age. The statistically significant odds ratio for high organic particle exposure: low birthweight (OR = 1.19; 95% CI: 1.07–1.32), small for gestational age (OR = 1.22; 95% CI: 1.07–1.38) or preterm birth (OR = 1.17; 95% CI: 1.08–1.27) compared to unexposed. The statistically significant odds ratio for high inorganic particle exposure: preterm birth (OR = 1.18; 95% CI: 1.07– 1.30), low birth weight (OR = 1.32; 95% CI: 1.18–1.48), and small for gestational age (OR = 1.20; 95% CI: 1.04– 1.39) compared to unexposed.

Subgroup analyses showed that the increased risks associated with exposure to inorganic particles were driven by exposure to iron particles. Subgroup analyses of the compounds in organic particle exposure showed an increased risk of small for gestational age associated with oil mist exposure. Exposure to oil mist and cooking fumes was also associated with low birthweight, and other organic dust and paper was associated with preterm birth. No increased risks were found in relation to exposure to stone and concrete particles. An increased risk of small for gestational age (OR = 1.40; 95% CI: 1.15–1.71) was associated with exposure to combustion products. Welding fumes were also analysed separately and high exposure was associated with an increased risk of low birthweight (OR = 1.22; 95% CI: 1.02–1.45) and preterm birth (OR = 1.24; 95% CI: 1.07–1.42). Residential exposure to combustion related particles (PM1) during the pregnancy was associated with a reduction in birthweight: Low exposure -15.3 g (95%CI: -19.4 g; -11.1 g), Moderate exposure -24.1g (95%CI: -28.4g;-19.8g), High exposure -29.0g (95%CI: -33.6g;-24.5g) compared to those in the very low exposed group. No clear interaction effect on fetal growth was found after exposure to both occupational and residential particles during pregnancy, but an additive effect was suggested. Weak associations were found between residential exposure to particles and low birthweight or small for gestational age, while no increased risk was shown for preterm birth. Only a marginal effect was seen when using occupational exposure as a confounder in the analyses on residential air pollution. Occupational exposure to organic (e.g. oil mist, paper dust, cooking fumes and other organic particles) and inorganic particles (e.g. iron particles), as well as combustion products (PAH) and welding fumes, have shown to be associated with an increased risk of adverse birth outcomes. The results strengthen the view that women should not be exposed to high levels of these particles during pregnancy, but the results needs to be confirmed in future studies. Separate effects on birthweight were seen after exposure to particles in occupational and/or residential setting. The results implied that an overall exposure assessment of particles for each individual is needed when assessing risks, but further studies are needed to confirm these findings.

List of scientific papers

I. Norlén F, Gustavsson P, Wiebert P, Rylander L, Albin M, Westgren M, et al. Occupational exposure to inorganic particles during pregnancy and birth outcomes: a nationwide cohort study in Sweden. BMJ Open. 2019;9(2):e023879.
https://doi.org/10.1136/bmjopen-2018-023879

II. Norlén F, Gustavsson P, Wiebert P, Rylander L, Westgren M, Plato N, et al. Occupational exposure to organic particles and combustion products during pregnancy and birth outcome in a nationwide cohort study in Sweden. Occupational and Environmental Medicine. 2019;76(8):537.
https://doi.org/10.1136/oemed-2018-105672

III. Norlén F, Gustavsson P, Wiebert P, Rylander L, Westgren M, Plato N, Albin M, Bennet C, Gidhagen L, Bellander T, Selander J. Residential air pollution during pregnancy and birth outcome in a nationwide cohort study in Sweden - is the association modified by occupational exposure to particles? [Manuscript]