How do I decide if I should swim in the Thames?

Simon Griffiths explains how he looks at pollution risk before swimming

The Great Stink happened in 1858. The Thames was so polluted that the smell drove parliament to relocate and introduce measures to clean up the mess. Those measures only provided temporary relief for in 1957, the Thames was declared biologically dead. Pollution was so bad, nothing could live there.

Today, the Thames thrives with life. Anglers line its banks. Herons and other fish-eating birds are common. Seals swim, and thrive, as far upstream as Teddington and beyond.

But, unless you’ve been living in a cave since the pandemic, you will be aware that not all is well with the Thames, or indeed any of England’s rivers. Water companies stand accused of putting pollution before profits, allowing their infrastructure to decay, and pouring untreated sewage into our rivers.

Some context here. Water companies have always been allowed to spill sewage into water courses during extreme rainfall events that overwhelm their systems. However, evidence collected by campaigners suggests they have discharged more sewage than permitted, causing long-term damage to rivers, and the plants and animals that depend on them.

We are instinctively repulsed by sewage. Encountering it in rivers or the sea disgusts us. As swimmers, it puts our health at risk. Given that many of us swim for health reasons, it raises the question: should we stop swimming in open water?

My answer is emphatically no.

Keep on swimming

I know I have a conflict here. I publish a magazine about outdoor swimming. It’s in my interest to encourage people to swim outdoors.

But it’s more than that. Swimming, and especially swimming outdoors, is an amazing activity. I don’t have numbers or hard evidence to back this up, but I believe the benefits far outweigh the risks of getting ill from pollution.

Second, swimmers were partly responsible for raising awareness of sewage pollution. Being a swimmer makes sewage pollution a threat to our identity as well as our health, which means we truly care. That passion drives action, which results in change. Politicians now have to push the water companies to improve because we’ve told them how much it matters and voted for the ones that promised change. If we stop swimming, it relieves the pressure.

Assessing the risk

All that said, I don’t advocate taking unnecessary risks with sewage pollution. I am selective about when and where I swim. But this requires making decisions with imperfect information and tolerating a degree of uncertainty.

In an ideal world, we’d have a device like a thermometer we could stick in the water that would show a green light if it was safe to swim. However, it’s unlikely we will ever have anything like this.

Traditional tests for sewage pollution measure E. Coli and Intestinal Enterococci bacteria. These depend on taking a sample and cultivating it overnight or longer. A result 24 or 48 hours after you want to swim isn’t much use and tells you nothing about conditions now.

Newer devices aim to estimate bacteria levels by proxy measures and give quicker results. Some are in use on the Thames and the results are shared on the Water Testing River Thames website. This is an improvement, but it still only gives you weekly data from a single point. You could have sewage discharging 200m downstream that won’t be detected.

Nevertheless, testing shows us that the Thames can, and frequently does, meet bathing water standards in multiple locations. It also shows us that bacteria levels can spike. This seems to happen after heavy rainfall, as we’d expect, but I’m yet to see a detailed analysis.

The other source of data I look at is Thames Water’s EDM map, which shows known combined storm overflows and provides live information when they spill. Surfers Against Sewage has an app that gives the same data for the coast.

No exact formula

It would be fun to have a formula to plug data into that would tell us if we could safely swim. Inputs might be: time since last sewage spill, distance from the spill, how long did the spill last, how fast is the river flowing, the water temperature, how long I will swim for and whether I will put my face in the water.

I don’t have such a formula, but these are the questions I ask myself before I swim. I also look for visual clues and note the smell. I factor in what I’m doing the next day. If I’m going to be travelling or doing a race, I’m more risk averse than if I’ve got a relaxed day at home. Then, how important is the swim? I might accept a higher risk for an event I’ve been looking forward to than a regular dip. I think too about the people I’m swimming with. Are they new to outdoor swimming and would getting ill put them off? Are they vulnerable or do they care for someone vulnerable?

After thinking through all of this, I essentially let my intuition guide me. If it feels wrong, I won’t swim. I also listen to the people I swim with. There is power in collective wisdom. I probably wouldn’t swim if they thought it was fine and I didn’t, but I would think twice and most likely cancel a swim, if I was ready to go but they weren’t.

Finally, I take the usual practical measures to reduce infection. I wear goggles to protect my eyes. I try not to let water into my mouth. I take a bottle of fresh water to rinse myself with when I finish swimming and I disinfect my hands before eating

With so many variables and personal factors, there will never be a perfect answer and there will also be some risk. But remember, if you do get ill, it will most likely be a short-term self-limiting illness. You will hear the occasional story about someone becoming seriously ill, but we hear these stories because they are rare.

You can’t guarantee your safety in open water, just as you can’t guarantee your safety on the road, but you can manage your risk and reduce the chance of being affected by pollution. With a bit of care, you can enjoy swimming outdoors and all the benefits without taking excessive risks.