# Exposure to ambient air pollution during pregnancy and risk of early-onset breast cancer

## TL;DR

This study finds that exposure to air pollution during pregnancy is linked to a higher risk of early-onset breast cancer.

## Contribution

The study is novel in examining air pollution exposure specifically during pregnancy and its association with early-onset breast cancer.

## Key findings

- Exposure to PMcoarse during the last pregnancy was associated with a 12% increased risk of breast cancer.
- Invasive breast cancer, but not ductal carcinoma in situ, was linked to air pollution exposure.
- Exposures during pregnancy periods showed stronger associations than exposures at age 35 or post-pregnancy.

## Abstract

Air pollution has been linked to breast cancer risk, but previous studies have seldom considered specific exposure windows, like pregnancy. During pregnancy the breast undergoes substantial changes and exposures may have a stronger impact than if they occurred during other time periods. This study aims to identify associations between ambient air pollution exposure during pregnancy and risk of early-onset breast cancer.

Using nationwide data from Swedish registers, we constructed a cohort consisting of all cancer-free women in Sweden giving birth to their first child between 1991 and 2015. Residential exposure to nitrogen dioxide (NO2), particulate matter < 10 μm (PM10) and < 2.5 μm (PM2.5) were modelled based on air pollution concentrations from 2019. Particulate matter between 2.5 and 10 μm (PMcoarse) was calculated separately. Detailed data on residential addresses (including exact moving dates) were available for the entire study period, allowing for spatial variation in the exposure dataset. Mean air pollution levels were assessed at first pregnancy, last pregnancy, 35 years of age, and 2 years after the last delivery. Associations were evaluated using Cox proportional hazards regression to estimate adjusted hazard ratios (HRs) and 95% confidence intervals (CIs).

Among 1,019,076 women, 12,085 (1.2%) were diagnosed with breast cancer and 65.2% moved at least once between their first pregnancy and two years after their last delivery. All exposures during pregnancy periods were positively associated with breast cancer, with the highest HR observed for exposure to PMcoarse during the last pregnancy (HRPMcoarse = 1.12 (95% CI = 1.04, 1.20) per 5 μg/m3 increase). The lowest HR were for NO2 levels estimated at 35 years of age, regardless of pregnancy status (HRNO2 = 1.03 (95% CI = 0.99, 1.06) per 10 μg/m3 increase). In analyses differentiating between invasive breast cancer and ductal carcinoma in situ, only invasive breast cancer was associated with air pollution exposure.

In this cohort study, air pollution exposure was consistently associated with increased risk of early-onset breast cancer.

The online version contains supplementary material available at 10.1186/s13058-025-02165-9.