Chlorine

Drinking water comes from a range of places, including lakes, rivers, and groundwater. But if you drink water straight from these sources there’s a risk that it could be contaminated with micro-organisms and make you sick.

What's a micro-organism?

Micro-organisms are living things that are so small they can only be seen using a microscope (not with the naked eye). Viruses and bacteria are two common types of micro-organisms. When people say things like ‘germs’ and ‘bugs’, they’re talking about micro-organisms.

But, there’s a safe, simple, inexpensive fix. It’s chlorine. And it’s been used to help disinfect community drinking water supplies for over 120 years.

Today, chlorine helps to keep drinking water safe for millions of people around the world.

In drinking water treatment plants that use chlorine, a very small amount is added to water. This kills most common bacteria and viruses. For example: campylobacter and norovirus.

After it leaves the treatment plant, drinking water travels through a network of pipes to get to the people who drink it.

During this journey, water can become contaminated. For example, if pipes become damaged. However, chlorine added when the water leaves the treatment plant continues to protect the drinking water by killing bacteria that enter the system before it reaches the people who drink it. We call this process: residual disinfection.

Chlorine: Part of a bigger picture for safe drinking water

No one thing is effective against all types of contamination.

While chlorine can kill bacteria and viruses in drinking water, it doesn’t protect against protozoa.

Protozoa are single-celled parasites, like cryptosporidium and giardia, that can make people sick. But they can be removed from water with filters or inactivated using ultraviolet light, so that they don’t pose a health risk.

So, while chlorine is critical, it usually needs to be used alongside other processes to make sure water is safe to drink.

We call this taking a ‘multi-barrier approach’ to managing risks. This approach is a key principle of drinking water safety and required under the Water Services Act 2021.

It means that water suppliers must use a range of processes, procedures, and tools that all work together to treat water from the source to the people that drink it. That way there’s always a range of safeguards in place to ensure drinking water stays safe.

Find out more about the multi-barrier approach to drinking water safety

DID YOU KNOW?

Chlorine is a common element in nature, where it’s usually combined with other elements.

The largest amount of chlorine on earth is found in the oceans as sodium chloride (salt). Salt and water are the most common ingredients used to make the chlorine used in your drinking water.

Chlorine can be added to drinking water in many forms, including as a gas, tablet, granule, or liquid.

Chlorine can also help suppliers find contamination

Monitoring chlorine levels can also help drinking water suppliers to find problems in their supply system.

Chlorine is used to keep drinking water safe as it travels from the treatment plant to your tap. This means that contamination in the system will “use up” available chlorine.

So, an unexpected drop in chlorine levels in a system could indicate a local source of contamination, which a drinking water supplier can act quickly to address.

Health and wellbeing

World Health Organisation studies confirm that the small amount of chlorine needed to treat drinking water is safe for people.

The amount of chlorine in drinking water when it leaves the treatment plant will be between 0.2 - 1.0 milligrams/litre. That’s means there’d be around one drop of chlorine in a standard bathtub (150 litres) full of water.

Some people with very sensitive skin, or skin conditions like eczema, could experience skin irritation after a shower or bath. But we expect this to be rare since an extremely small amount of chlorine is added to drinking water. If you experience skin irritation, you can:

call Healthline on 0800 611 116 any time for free health advice
talk with your GP.

Disinfection by-products

Chlorine can react with organic material in some drinking water supplies to form by-products, like trihalomethanes. There is no clear evidence that these by-products impact your health.

The Drinking Water Standards for New Zealand provide maximum acceptable values for common disinfection by-products. The World Health Organisation makes it clear that while steps should be taken to control these chemicals, effective disinfection is critical and must not be compromised.

Taste and smell

While most people don’t mind the taste and smell of chlorinated water, some people can taste the low levels of chlorine in drinking water.

To remove the taste and smell, you can keep drinking water in an open jug in the fridge. In a few hours the chlorine taste will dissipate naturally. You could also use an activated carbon water filter or simply pour water from one jug to another several times.

Pets and fish

Water treated with chlorine is safe to drink for cats, dogs, and other mammals. It’s also safe to drink for birds.

Chlorine can be toxic to fish, other aquatic animals, reptiles, and amphibians. That’s because unlike people and other household pets, these types of animals absorb water directly into their blood stream.

For these pets, chlorine can be removed from water by letting it sit out for a few days or by buying a product at your local pet store that removes the chlorine. Ask your local pet store for more information.

The rules for drinking water suppliers in Aotearoa

As the drinking water regulator for Aotearoa New Zealand, we’re responsible for administering requirements for drinking water safety.

You can find these in the :

These standards, rules and other documents set out detailed requirements that complement the broader duties in the Water Services Act 2021 and help to ensure drinking water safety.

The Act requires water suppliers to provide residual disinfection in pipe networks. This means that a safe disinfectant, like chlorine, must be added to keep the drinking water safe from contamination as it travels from the treatment plant to the people that drink it.

Chlorine is most commonly used for residual disinfection because it’s easy to access, affordable and effective against most micro-organisms (like viruses and bacteria).

The history of chlorine and drinking water

1744: Chlorine was first discovered in Sweden.
1890: Chlorine was found to be an effective tool to use to disinfect and reduce the amount of sickness spread through unsafe drinking water.
1897: A bleach solution was used to disinfect a water main in Maidstone, Kent following an outbreak of typhoid.
1905 - 08: Great Britain and the United States start adding chlorine to water supplies, which improved public health. For example, it supported a dramatic drop in typhoid deaths.
1917: Canada starts adding chlorine to community water supplies to improve public health.
Today: Chlorine, or chlorination, is the world’s most widely used tool for disinfecting drinking water. It’s affordable and effective against most micro-organisms (like viruses and bacteria) found in water.

Slightly elevated arsenic levels in the Waikato River

For the public