The rapid increase in the number of urban inhabitants will be among the most important global health issues of the 21st century. Urban living is becoming more predominant and city dwellers are facing many new health challenges.Urbanization is closely associated with the scarcity of clean water, excessive violence and traffic accidents, and an increased exposure to risk factors such as tobacco use, unhealthy diets, physical inactivity and harmful use of alcohol.Environmental risk factors contribute to 85 of the 102 major diseases covered by the World health report. Approximately 23 per cent of all deaths can be attributed to environmental factors – many of which could be prevented.The greatest absolute disease burden attributable to modifiable environmental factors included diarrhoea, lower respiratory infections, ‘other’ unintentional injuries and malaria.Children represent our future and all children need a healthy, safe and protective environment to ensure normal growth, development and overall well-being. In children under the age of five, one third of all disease is caused by the environmental factors such as unsafe water and air pollution.Unsafe drinking water, poor sanitation and hygiene are the strongest links to diarrhoeal disease – a leading childhood killer. Lower respiratory infections are often associated with indoor air pollution related to household solid fuel use and second-hand tobacco smoke, as well as outdoor air pollution.Asthma is the most common chronic disease among children and is triggered by environmental factors such as house dust mites, second-hand smoke, moulds and pollens. Reducing exposure to environmental triggers can effectively control asthma.The main sources of environmental air pollution are from industries such as power stations and emissions from agriculture. Fossil fuel emissions from cars and trucks have skyrocketed in recent years with rapid urbanization and the increased reliance on motorized transport of people and goods.Environmental air pollution also includes smoke and emissions from burning waste dumps, rubbish, firewood and charcoal. These activities occur in and around the home and are major causes of respiratory disease in both adults and children.Environmental factors, such as inadequate pedestrian and cycling infrastructures, also make a significant contribution to physical inactivity levels and injuries associated with road traffic accidents.It has been estimated that physical inactivity levels could be reduced by 31 per cent through improved environmental interventions, including pedestrian- and bicycle-friendly urban land use and transport, leisure and workplace facilities, and policies that support more active lifestyles.Healthier environments can also reduce the incidence of noncommunicable diseases such as cancer, cardiovascular disease, diabetes and overweight and obesity. Up to 80 per cent of heart disease, stroke and type 2 diabetes could be prevented and 40 per cent of cancer."There’s also a lot we can do as individuals to lower our chances of developing the disease such as being more physically active and adopting a healthier diet,” says Dr Rachel Thompson, head of research interpretation at the World Cancer Research Fund International.Recent reports from the National Cancer Institute found that fewer than 5 per cent of adults get the recommended 30 minutes of moderate exercise per day.Physical activity and exercise is needed for all – regardless of weight, health condition or age – to achieve optimal health and fight off disease. Dr Cory Couillard is an international health columnist that works in collaboration with the World Health Organization’s goals of disease prevention and control.  Facebook: Dr Cory CouillardTwitter: DrCoryCouillard