Women living and working in the city have a higher risk of breast cancer, researchers say.
Researchers said breast screening was key
The study of 972 women by London's private Princess Grace Hospital found city women had much denser breasts.
Previous research has shown those with the densest breast were four times more likely to develop cancer.
Researchers, presenting the study to the Radiological Society of North America, said air pollution was likely to be the cause of denser breasts.
It is thought air pollution contains tiny particles that mimic female sex hormones and can disrupt the make up of breasts.
Professor Kefah Mokbel, one of the researchers, said: "Women who live or work in cities should pay greater attention to breast screening. Ironically, uptake of breast screening is lower cities like London than in the countryside.
"The other implication is that we have to focus on better traffic management and find ways to reduce traffic emissions.
"This is an important issue for the public and politicians, and it's vital that we raise awareness of the link we have found."
Breasts are partly composed of fat and partly "dense" glandular tissue.
Previous research has shown that women with 25% or more of their breasts made up of dense tissue are at significantly higher risk of breast cancer than those with fattier breasts.
One reason is that tumours in dense tissue are difficult to spot using X-rays.
There is also evidence that cancers are more likely to develop in dense breasts.
The new research, which analysed mammograms of women who had taken part in breast screening, showed that city women aged 45 to 54 were more than twice as likely to have at least 25% of their breasts made of dense tissue as those from the countryside.
Generally the trend was most obvious in women under the age of 50 and those working in London's Square Mile.
Researchers also warned the stress of city life could also be putting them at increased risk.
But Professor Stephen Duffy, Cancer Research UK's professor of screening, said the findings may be related to weight.
"The Health Survey for England found that women living in London were the thinnest in the country, and breast density is known to be inversely related to body weight."
But he added whatever the reasons, the study did demonstrate the need for careful attention to breast screening as "greater breast density makes mammography a more challenging job".