Waterborne diseases are caused by pathogens (disease-causing microorganisms, such as viruses, bacteria and protozoa) spread through contaminated drinking water or recreational water.

Climate change could cause an increase in the incidence of water-borne illnesses in a number of ways. Most of the viruses, bacteria and protozoa that cause water-borne diseases thrive in warm water and weather. Therefore, increased water and air temperatures could stimulate the growth of harmful pathogens. In addition, increased rainfall events can lead to these pathogens being deposited in water, thereby leading to contamination.

Water can become contaminated by pathogens in several ways, the most common being the improper disposal of sewage and waste. The majority of waterborne microorganisms that cause human disease come from animal and human faeces. Sewage, or wastewater, is not supposed to be released raw and untreated into the environment; however, such release may happen inadvertently in communities that use combined sewage and storm water drainage systems. The increase in volume of water following a heavy rainfall event can overwhelm a treatment plant’s capacity. In such cases the overflow is discharged — with limited or no treatment — directly into surface water bodies.

Water can become contaminated through surface runoff during a heavy rainfall, which can allow pathogens to find their way into aquifers, wells and drinking water.

Water can also become contaminated with natural toxins produced by algae. This happens when warm coastal waters combine with nutrient rich (i.e., fertilizer and sewage) runoff to produce expansive blooms, or “red tides,” of toxic algae or cyanobacteria.” These algae can carry potent nerve toxins that are very harmful to humans.

Drought can also play a role in water contamination. During a drought period   there is less runoff flowing into lakes, ponds and streams. This can lead to low water levels, which means that less water is available to disperse and dilute pollutants. Low water levels also mean higher temperature water and increases in the potential for algae growth.

Adapted from Pollution Probe's Climate Change and Human Health Primer