The effects of pollution on reproduction are felt globally. Pollution significantly interferes with all aspects of sexual reproduction from male and female fertility to fetal development, birth outcomes, and long-term health of offspring. The risk of adverse birth outcomes such as miscarriage, stillbirth, preterm birth, low birth weight and birth defects increase when pregnant women are exposed to air pollution.
The criteria air pollutants sulfur dioxide, nitrogen dioxide, particulate matter, carbon monoxide, ozone, and lead have been identified as harmful to human health by the World Health Organization and the U.S. Environmental Protection Agency (EPA) and are known to interfere with reproduction. Many of them are linked to the burning of fossil fuels, through vehicle use, heating and cooking. Indoor and outdoor air pollution can also contain endocrine-disrupting chemicals and microplastics that disrupt fertility, fetal development, and reproductive health.
Pollutants can be inhaled, eaten, drunk, or absorbed through skin. Steps can be taken to minimize exposures such as avoiding smoking, minimizing dust, using glass instead of plastics, carefully selecting cleaning products and cosmetics, and exercising away from heavy traffic routes. Control strategies can also be implemented to improve industrial safety and public health. Cities such as London, Paris, Bogotá and Kraków have made changes that decrease pollution.
Demographics Total fertility rates (TFRs) have fallen globally in all recorded countries since the early 1960s. It has been argued that cultural and social factors are not sufficient to explain this global decrease. For example, fertility rates in China were already falling before the introduction of social campaigns such as the "later, longer, fewer" campaign (1973–1979) or the one-child-family policy (introduced in 1979). The United Nations reported in 2022 that two-thirds of the global population live in countries with fertility rates below replacement levels.
The reduction in fertility rate over the last fifty years correlates with increasing global prosperity, but also with increasing environmental pollution. Given that all countries seem to be affected, it has been argued that environmental contamination may explain the widespread decline in fertility. As of 2022, the World Health Organization reported that 99% of the world’s population lives in places with unhealthy levels of PM2.5 pollution. Environmental contaminants have been shown to impact human fertility in a variety of ways.
While populations worldwide are reporting lowered reproductive ability, marginalized communities and minorities tend to be exposed to higher levels of air pollutants than majority populations. They experience higher rates of adverse reproductive outcomes. The impact of race and ethnic group was examined in a data meta-analysis of perinatal outcomes from 53 high- and upper-middle-income countries. Black women were consistently at higher risk for adverse perinatal outcomes including neonatal death, stillbirth, preterm birth, and small-for-gestational-age babies compared to white women. South Asian women were also more likely to experience adverse outcomes such as preterm birth and small-for-gestational-age babies, irrespective of geographical region. Reproduction is increasingly being examined within a framework of environmental health and reproductive justice.
Pollutants Pollutants can be inhaled, eaten, drunk, or absorbed through skin, with inhalation being the most common means of exposure. Air pollution is a volatile mixture of both gases and particles whose components differ in terms of chemical reactivity, duration of exposure, and severity of effects. Pollutants can travel long distances, and mixtures of pollutants can vary widely across both space and time depending on factors such as pollution sources, geography, and season. Such differences can make it difficult to generalize across studies. Research done in different places or at different times may attribute the effects of pollution to different components based on the mixture of pollutants present.
Measurement of individual components within mixtures and estimation of their causal effects are complicated by the presence and interactions of multiple components. In cases where a single pollutant is used as a proxy indicator for general levels of air pollution, observed effects may be misattributed to the measured pollutant, when unmeasured components of polluted air are actually responsible. Mixture analysis methods are used to measure and examine multiple components and better attribute the overall effects of air pollution to specific components.
Research has consistently shown that exposure to pollutants can affect fertility, fetal development, health of offspring in later life, and the genetics of future generations. Potential mechanisms of action by toxic environmental chemicals include oxidative stress, inflammation, changes in placental function, endocrine disruption and genetic alterations.
Carbon monoxide Carbon monoxide (CO) is an odorless gas released by the burning of fossil fuels in fires, boilers and motor vehicle engines. CO is also released by biomass fueled-cooking and is found in firsthand and secondhand tobacco smoke. CO affects both outdoor and indoor air quality. CO from traffic-related air pollution is found in higher concentrations in areas of higher traffic volume and congestion, and in poorly ventilated traffic and parking areas.
Carbon monoxide binds to hemoglobin in red blood cells, reduces the oxygen-carrying capacity of blood, and interferes with cellular respiration. It prevents delivery of oxygen to tissues and causes cellular and tissue damage. In cases of pregnancy, the effects of carbon monoxide poisoning are much greater and longer lasting on the fetus than on the pregnant woman.
Nitrogen oxides Nitrogen oxides (NOx) are a group of highly reactive gases that contain nitrogen and oxygen. They form when fossil fuels are burned at high temperatures. Sources include heavy machinery and other motor vehicles, power generation, and a variety of industrial, commercial, and residential activities. As pollutants, NO, NO2, and other nitrogen oxides are associated with increased risk and a wide range of negative health outcomes, including reproductive harms.
In the body, nitric oxide (NO) normally acts as a signaling molecule, playing a central role in physiological processes that include vasodilation, inflammation, immune responses, cell death, and gene regulation. NO is essential to the synthesis and secretion of reproductive hormones including follicle-stimulating hormone, progesterone, estrogen, luteinizing hormone, gonadotrophin, and prostaglandin. Many stages of reproduction in males and females require highly specific NO levels. In males, NO regulates key functions related to sperm motility, hyperactivation, capacitation, and maturation. In females, NO affects follicle development, menstruation, maturation of oocytes, ovulation, fertilization, implantation, embryo development, successful maintenance of pregnancy, and labor.
As pollutants, nitric oxide (NO) and nitrogen dioxide (NO2) easily enter the body via the lungs and circulatory system. They can disrupt the highly sensitive activities of endogenous NO and cause harms through mechanisms such as oxidative stress, endocrine disruption, placental inflammation and the alterations in the development of stem cells or precursor cells.
Ozone
Ground level ozone is a gas composed of three atoms of oxygen. As an air pollutant, ozone may be linked to negative reproductive outcomes. However, its effects on female fertility are not entirely clear. Under some conditions, ozone may have anti-inflammatory effects.
There is evidence that the combination of ozone with PM2.5 amplifies harmful effects such as oxidative stress, inflammatory processes, loss of membrane integrity, and reproductive dysfunction. Ozone may also interact with NOx to create more serious effects than would occur separately.
Sulfur dioxide Sulfur dioxide (SO2) is colorless, highly reactive toxic gas. Its primary source is the burning of sulfur-containing fuels, which converts sulfur in the fuel to SO2. The SO2 derivatives bisulfite and sulfite are widely used as food additives.
SO2 exposure is linked to decreased sperm quality. SO2 also has been linked to decreases in antral follicle count (AFC), resulting in diminished ovarian reserves in women. SO2 reduces DNA synthesis and may interfere with cellular repair mechanisms and increase risk of miscarriage.
Particulate matter
Particulate matter is made up of microscopic particles of solid or liquid matter suspended in the air. Inhalable coarse particles, called PM, have a particle diameter of 10 micrometers (μm) or less. Fine particles, called PM, have a diameter of 2.5 µm or less and are able to penetrate deeper into the lungs. Sources of particulate matter include desert dust, smoke from fires, first and second-hand tobacco and cannabis smoke, and automobile exhaust fumes from fossil fuel combustion.
PM has been shown to disrupt hormone levels and may affect placental health. It may contribute to reproductive toxicity through deterioration of the air-blood barrier and mechanisms of oxidative stress, inflammation, DNA damage, and cell death.
Polycyclic aromatic hydrocarbons
Polycyclic aromatic hydrocarbons (PAHs) are toxic, semi-volatile chemicals that are created by combustion of organic materials. PAHs such as benzo(a)pyrene (BaP) are found in exhaust fumes, cooking smoke, and tobacco smoke, among other sources.
PAHs are associated with reduced fertility. They have been reported to administer their toxic effects through oxidative stress by increasing the production of Reactive Oxygen Species (ROS) which can contribute to inflammation and cell death. Long-term exposure to PAHs can result in DNA damage and reduced repair.
Endocrine-disrupting chemicals Endocrine-disrupting chemicals (EDCs) are a group of toxicants that include PCBs, PFAS, PBDEs, parabens, solvents, Triclosan, dioxins, pesticides, phthalates, heavy metals and Bisphenol A. EDCs have become ubiquitous environmental pollutants due to their widespread agricultural, industrial, and consumer use. EDCs can be found in commonly used cleaning and personal care products, in plastics, and in e-cigarettes. EDCs can be present in both outdoor and indoor air, as part of particulate matter, and in gaseous form as volatile organic compounds (VOCs) and semi-volatile organic compounds (SVOCs).
EDCs interfere with the body's endocrine system. They can mimic, block, or alter the effects of naturally-occurring hormones in the body that are critical to reproduction and development. Endocrine disruption can cause hormonal changes, increased oxidative stress, and genetic damage. They affect gametes, embryos, gonads, and reproductive tract development and function, and have been shown to interfere with fertility, fetal development, and reproductive health.
Microplastics
Microplastics (MPs) are typically defined as plastic fragments between 1 µm and 5 mm in diameter. Microplastics have become pervasive environmental contaminants, found in freshwater and marine systems, soil, indoor dust, and air. A major source of microplastic pollution is the breakdown of larger plastic debris, including the packaging of foods and beverages. Microplastics are also found in many personal care products, synthetic textiles, and industrial waste. A study of placentas from pregnancies in Hawaiʻi found substantial increases of microplastics. They were found in 60% of placentas in 2006, and in all placentas in 2021.