Although the pesticide DDT was banned over 20 years ago, studies have shown that there are still trace amounts found in certain agricultural products, as well as in human and animal milk. While individual studies have come to conflicting conclusions, the most recent reviews of all the evidence conclude that exposure to DDT before puberty increases the risk of breast cancer later in life.

Ethylene oxide

Ethylene oxide is a chemical that can be found in some personal care products, mainly in the form of fragrance. It is also used for the sterilization of various medical objects. The National Toxicology Program has labeled ethylene oxide as a definite human and animal carcinogen. A study done by the National Institute for Occupational Safety and Health including 7,576 women found a direct correlation between breast cancer rates and exposure to ethylene oxide during medical sterilization processes. Also, human breast cells put into contact with small amounts of ethylene oxide in a laboratory can lead to DNA damage of the breast tissue.

Polycyclic aromatic hydrocarbons

Polycyclic aromatic hydrocarbons are chemical products of combustion from coal burners, fuel, cigarette smoke, and various other sources. PAH's are often found in the air and are breathed into the body. PAH's bioaccumulate easily and can copy the estrogen hormone. PAH's can also be genotoxic, meaning they have the ability to harm DNA.

Vinyl chloride

Vinyl chloride is produced when PVC or polyvinly chloride is made. PVC is found in plastic packaging, outerwear, plastic toys and other plastic products. Vinyl chloride can be found in cigarette smoke and the air around garbage and land fills. It can also be found in the wastewater when PVC is made. The National Toxicology Program and the International Agency for Research on Cancer have both labeled vinyl chloride as a definite human carcinogen.

Tobacco

Until recently, most studies had not found an increased risk of breast cancer from active tobacco smoking. Beginning in the mid-1990s, a number of studies suggested an increased risk of breast cancer in both active smokers and those exposed to secondhand smoke compared to women who reported no exposure to secondhand smoke. By 2005 enough evidence had accumulated for the [California Environmental Protection Agency] to conclude that breathing secondhand smoke causes breast cancer in younger, primarily premenopausal women. The Agency concluded that the risk was increased by 70%, based on epidemiological studies and the fact that there are many mammary carcinogens in secondhand smoke. The following year (2006) the US Surgeon General identified the same risk increase and concluded that the evidence is "suggestive," one step below causal. There is some evidence that exposure to tobacco smoke is most problematic between puberty and first childbirth. The reason is that breast tissue appears most sensitive to chemical carcinogens breast cells not fully differentiated until lactation. The likely reason that the older studies of active smoking did not detect risks associated with smoking was that they compared active smokers to all nonsmokers (which includes many passive smokers). The newer studies, which exclude passive smokers from the control group, generally show elevated risks associated with active as well as passive smoking.

Passive smoking

Breathing secondhand smoke increases breast cancer risk by 70% in younger, primarily pre-menopausal women. The California Environmental Protection Agency has concluded that passive smoking causes breast cancer and the US Surgeon General has concluded that the evidence is "suggestive," one step below causal. There is some evidence that exposure to tobacco smoke is most problematic between puberty and first childbirth. The reason that breast tissue appears most sensitive to chemical carcinogens in this phase is that breast cells are not fully differentiated until lactation.

Radiation

Women who have received high-dose ionizing radiation to the chest (for example, as treatments for other cancers) have a relative risk of breast cancer between 2.1 to 4.0. The risk increases with increased dose. In addition, the risk is higher in women irradiated before age 30, when there is still breast development.

Dioxins

Dioxins (most notably the polychlorinated dibenzodioxins) are chemicals that are produced when chlorinated products are burned, such as polyvinyl chloride (PVC). This occurs when chlorinated products are used in certain manufacturing industries. Dioxins are also added to the air when gasoline and diesel fuels break down. Dioxins are able to bioaccumulate, meaning that they settle and stay in human and animal fat for long periods of time. There are many different types of dioxins and only a few of them have been labeled by the Environmental Protection Agency as definite human carcinogens and endocrine hormone disruptors. Although dioxins are floating in the air, they eventually settle on plants and other vegetation surfaces. These plants and vegetation are them eaten by cows and other animals. Humans end up eating the produce, milk, eggs, and meat produced by these animals that have consumed dioxin covered vegetation. Dioxins are more harmful when ingested this way. Multiple studies have led to the idea that increased dioxin levels can increase one's risk for breast cancer. A study done in 1976 after a chemical plant explosion in Seveso, Italy concluded that high dioxin level exposure in a woman's body correlated with a more than double chance of developing breast cancer.

Light at night and disturbance of circadian rhythm

In 1978 Cohen et al. proposed that reduced production of the hormone melatonin might increase the risk of breast cancer and citing "environmental lighting" as a possible causal factor. Researchers at the National Cancer Institute (NCI) and National Institute of Environmental Health Sciences conducted a study in 2005 that suggests that artificial light during the night can be a factor for breast cancer by disrupting melatonin levels.

In 2007, "shiftwork that involves circadian disruption" was listed as a probable carcinogen by the World Health Organization's International Agency for Research on Cancer. (IARC Press release No. 180). Multiple studies have documented a link between night shift work and the increased incidence of breast cancer. A review of current knowledge of the health consequences of exposure to artificial light at night including the increased incidence of breast cancer and an explanation of the causal mechanisms has been published in the Journal of Pineal Research in 2007.