Worldwide, unsafe drinking water is a major problem: one billion people do not have access to safe water.  While the occurrence of waterborne diseases in developed countries is usually low due to widespread good systems of water treatment, distribution and monitoring, waterborne diseases are amongst the leading causes of morbidity and mortality in low and middle income countries, frequently called developing countries.  For example, an estimated 900 million people suffer (and approximately 2 million die) from water-related illnesses each year.  At least 10 percent of the total burden of human diseases in many developing countries can be attributed to diarrhea and infestations by intestinal worms.

Contaminated drinking water is one of the leading sources of health problems for international travellers and expatriates.  Each year more than 10 million people going from an industrialized nation to a less developed country suffer diarrhea or more severe waterborne diseases. 

Most of the information sites for travellers recommend avoiding drinking water anywhere abroad.  Under no circumstances is it advisable to drink unsafe water, use it for oral hygiene or use ice made from unsafe water in drinks.  However, you should not become hysterical about avoiding any water whilst travelling abroad.   Santiago (Chile), has tap water as safe as the equivalent of any city in the developed countries; but in Freetown (Sierra Leone), it could be very dangerous to drink the  tap water, even if you can get it.  Just obtaining the most reliable information you can about water safety, becomes the best prevention.

In order to offer reliable information Safe Water for International Travellers , an independent organization, has prepared a Worldwide Rating of  Drinking Water Safety, a qualification that gives a broad indication of the health risks related with drinking water, to which international travelers or expatriates may be exposed in every country of the world.  All the countries of the world have been classified into five categories of drinking water safety:

5 Drops Countries
Where all cities have safe water supplies, which are monitored and regularly tested by an independent and qualified board, and drinking water quality is the standard in developed countries.  Waterborne diseases are almost non-existent.  Only about 20% of the world countries have this top classification.

4 Drops Countries
Where main cities have generally safe water supplies, which are monitored and regularly tested by an independent board, but level of service of rural and small towns systems could be substandard.  Prevalence of waterborne diseases is low.

3 Drops Countries
Where the level of service quality varies considerably between urban and rural residences, and between regions.  Some systems comply with World Health Organization-WHO Guidelines, but many of them have intermittent service and lack of disinfection.  Monitoring and testing is not fully reliable. Some waterborne diseases are endemic.

2 Drops Countries 
Where some water supplies comply with WHO Guidelines, but most of them do not. There is no permanent monitoring and testing.  Usually water quality is not good.  Prevalence of waterborne diseases is high.

1 Drop Countries
Where it is almost impossible to find drinking water that complies with WHO Guidelines. There is no monitoring and testing.  Waterborne diseases are the main cause of infant mortality.  Usually it is dangerous for foreign visitors to drink tap water.

A sample of the Rating list  is shown in the table below:

You can see the complete listing of the Drinking Water Safety Rating for every country of the world, in Safe Water for International Travellers website (http://www.safewateronline.com) as well as download it in pdf format.  Also you can get  more information about water supply characteristics, waterborne disease prevalence and its prevention, for the country you are interested in visiting. To assess drinking water quality is among the most difficult problems on a nation-wide basis, due to the lack of available data and the magnitude and complexity of measuring drinking water quality every time and everywhere. In practice, to define the degree of risk likely in each country through direct indicators is extremely difficult.  - Article continued below -