Can you get £71,000 on benefits?

Summary

More or Less — "Can You Get £71,000 on Benefits?"

BBC Radio 4 | Host: Tim Harford | Date: January 28, 2026

---

Episode Overview

In this episode, Tim Harford and the More or Less team tackle prominent claims and numbers that have captured public attention and political debate. The main theme revolves around the hotly contested assertion that a family on benefits can receive the equivalent of a £71,000 annual pre-tax salary—a figure wielded in political discussions about the UK benefits system. The episode also features segments fact-checking bold statements from public figures and pop culture, and a light-hearted physics exploration on saunas. The podcast ends on a personal note, as Tim embarks on a marathon challenge.

---

1. Debunking the £71,000 Benefits Claim

[01:03–08:13]

Background of the Claim

• Claim: "You'd need a £71,000 pre tax household income to be as well off as a couple with children on benefits" ([02:00] Richard Varden, quoting the Centre for Social Justice).
• This figure suggests working families need almost twice the median UK income to match what some out-of-work households may receive in benefits.

Origins and Context

• Sourced from a December report by the Centre for Social Justice—founded by former Conservative leader Sir Iain Duncan Smith. The report was a response to government changes (specifically, the scrapping of the two-child cap on Universal Credit by Chancellor Rachel Reeves) ([02:45–03:16]).
• The claim compares a single-earner family of five’s income to a benefits-receiving household where one adult is out of work due to illness/disability.

How Accurate Is That Figure?

• Pre-tax vs. Take-home Pay: The £71,000 figure reflects gross income, but after standard deductions (tax, NI, student loan, pension), actual take-home pay is "just over £46,000" ([03:35] Richard Varden).
• Benefits Cap: Most out-of-work families with three children would get about £25,000 (London) or £22,000 (elsewhere) due to the benefits cap ([04:51] Richard Varden).
• The exception: households where someone claims disability or illness benefits (not subject to the cap). In these cases, health elements and Personal Independence Payment (PIP) can raise total annual benefits to around £46,000 ([05:54] Richard Varden).

Is the Comparison Fair?

• Experts argue the comparison is misleading.
  • "The whole comparison is unfair, comparing a single person without disabilities against a family with children and rent and disabilities and all the costs that go along with them." ([06:24] Gareth Morgan via Tom Coles)
• Disability benefit is difficult to obtain; it's not simply a matter of applying.
  • "You have to prove your disability. And that's a very, very lengthy and onerous process." ([07:22] Tom Coles)

Are There Incentives to Claim More Benefits?

• Jo Shallum from the Centre for Social Justice insists the system does create incentives.
  • "If you are able to essentially double your benefit income by going through the gateway that says you are unable to work due to anxiety and/or depression... we have to treat that as a very real incentive." ([06:45] Tim Harford paraphrasing Jo Shallum)
• Evidence for this is limited. The IFS found that reducing the benefit cap in 2016 increased disability claims by just about 2%.
  • "The effect was not big, about a 2% increase..." ([07:40] Richard Varden)

Notable Quotes

• "[The] incentive to be disabled is a ridiculous kind of concept in the first place." ([07:09] Tom Coles, quoting Gareth Morgan)
• "It's a lot of money. But there is more to this comparison than meets the eye." ([03:16] Richard Varden)
• "You have to prove your disability. And that's a very, very lengthy and onerous process." ([07:22] Tom Coles)

---

2. Fact-Checking Political and Social Claims

A. Mark Carney and GDP Figures

[08:13–10:11]

• Former Bank of England governor Mark Carney claimed at the World Economic Forum that "Nordics plus Canada" make up 20% of global GDP.
  • Reality: Canada is about 2% and the Nordics ~3.5%; combined far less than 20% ([09:08] Tim Harford).
  • Carney meant "Canada plus EU," which actually does total 20%.

B. Right Said Fred and “Crimes Against Churches”

[10:41–16:14]

• Claim: "9,000 crimes against churches in the UK in the last three years."
  • The 9,000 figure actually includes all places of worship—mosques, synagogues, temples—not just Christian churches ([12:11] Lizzie McNeil).
  • "A lot of police forces include all places of worship within their count... so mosques, Buddhist temples and synagogues will be included." ([12:11] Lizzie McNeil)
• Types of crimes: The majority are thefts and burglaries, not hate crimes.
  • "There were over 3,000 cases of criminal damage and 2,102 cases of violence." ([13:09] Lizzie McNeil)

Religious Hate Crimes in the UK (Year ending March 2025)

• 7,000+ hate crimes recorded.
  • 45% against Muslims, 29% Jews, only 5% against Christians ([14:54] Tim Harford / Lizzie McNeil)
• Arson at places of worship does occur, but systematic anti-Christian arson is not evidenced.

Notable Quotes

• "Drug trafficking might be a crime committed in or near a church, but it would be weird to describe it as a crime against a church." ([13:32] Tim Harford)
• "Christians are at risk of violence in some countries around the world, but thankfully there is currently very little evidence that Christians in the UK face any real threat of violence." ([16:14] Lizzie McNeil)

---

3. Sauna Temperatures: 'Ten Times as Hot as Wales'?

[17:25–21:39]

Origin of the Claim

• On Radio 4’s Inside Health, it was said that saunas (80°C) are "ten times as hot" as wintery Wales (8°C).

Analysis

• Problem: Celsius (and Fahrenheit) are relative scales—not absolute measures of thermal energy, so multiplying doesn't make sense.
  • "[If] you convert to Fahrenheit... it's only four times warmer." ([19:20] Tom Coles)
• Correct Calculation: Use absolute zero (Kelvin scale)—80°C is 353.15K; 8°C is 281.15K.
  • "The ratio of the two of these is actually 1.256. That's about 1.26 times warmer." ([20:45] Lizzie McNeil)
• To be truly "ten times as warm" as 8°C, the sauna would need to be 2,500°C.

Notable Quotes

• "For the scale that you've chosen... zero degrees Celsius is not as cold as it can get. So it's at that point that the math starts to break down." ([18:56] Prof. Kelly Morrison)
• "If you are respecting physics, then a sauna is only 1.26 times warmer than outdoor Wales in winter." ([21:14] Tim Harford)

---

4. Marathon Science – ‘Breaking Four’

[21:39–29:16]

Tim Attempts to Break 4 Hours at the London Marathon

• Inspired by Kipchoge’s two-hour marathon feat, Tim aims for a sub-4 hour finish.

Science of Endurance

• VO₂ Max: The peak oxygen uptake—a cap on aerobic endurance ([23:48] Dr. Danny Muniz).
• Lactate Threshold: The % of VO₂ Max sustainable for a marathon; determines how hard you can "push" without exhaustion.

Lab Testing

• Tim is tested at the University of Hertfordshire: treadmill running, repeated blood lactate measurements.
• Result: "We did estimate your likely [lactate] threshold to happen at around 11 km per hour... That corresponds to a marathon time of 3 hours and 50 minutes." ([28:31] Dr. Danny Muniz)
• Factors like nutrition, sleep, and weather affect race-day outcome—if all goes well, Tim could "go under four hours." ([28:58] Dr. Danny Muniz)

Notable Moments

• "[VO₂] max essentially sets the ceiling of your aerobic metabolism." ([24:12] Dr. Danny Muniz)
• "I've got a charity place in the London Marathon... and I've got my eye on a target that is surely just as impressive." ([21:39] Tim Harford, tongue-in-cheek)

---

Memorable Quotes & Moments by Timestamp

• "[The] incentive to be disabled is a ridiculous kind of concept in the first place."
  — Gareth Morgan ([07:09])
• "You have to prove your disability. And that's a very, very lengthy and onerous process."
  — Gareth Morgan ([07:22])
• "Drug trafficking might be a crime committed in or near a church, but it would be weird to describe it as a crime against a church."
  — Tim Harford ([13:32])
• "If you are respecting physics, then a sauna is only 1.26 times warmer than outdoor Wales in winter."
  — Tim Harford ([21:14])
• "We did estimate your likely threshold to happen at around 11 km per hour... That corresponds to a Marathon time of 3 hours and 50 minutes."
  — Dr. Danny Muniz ([28:31])

---

Key Timestamps for Segments

• Intro & Topics Preview: 01:03
• Benefits Claim Analysis: 02:00–08:13
• Mark Carney GDP Fact Check: 08:13–10:11
• Right Said Fred Church Crime Claim: 10:41–16:14
• Sauna Temperature Comparison: 17:25–21:39
• Marathon Science and Tim's Challenge: 21:39–29:16

---

Conclusion

This fact-filled finale of More or Less explains why big headline numbers about benefits rarely translate into simple truths and exposes the problems of making apples-to-oranges comparisons in public debate. The team brings their signature wit and clarity to everything from social media myths to the physics of extreme heat, and wraps up with a personal experiment in marathon science. The episode exemplifies why careful, critical analysis is so needed—whether talking about government budgets, global GDP, or how many times warmer a sauna really is.

---

For feedback or questions, email [e-mail]

Transcript

[Tim Harford]

This BBC podcast is supported by ads outside the UK. The best B2B marketing gets wasted on the wrong people. So when you want to reach the right professionals, use LinkedIn ads. LinkedIn has grown to a network of over 1 billion professionals, including 130 million decision makers. And that's where it stands apart from other ad buyers. You can target your buyers by job title, industry, company role, seniority, skills, company revenue, so you can stop wasting budget on the wrong audience. It's why LinkedIn Ads generates the highest B2B return on ad spend of major ad networks. Spend $250 on your first campaign on LinkedIn Ads and get $250 credit for the next one. Just go to LinkedIn.com Broadcast. That's LinkedIn.com Broadcast. Terms and conditions apply.

[Lizzie McNeil]

Early birds always rise to the occasion for summer vacation planning because early gets you closer to the action. So don't be late. Book your next vacation early on VRBO and save over $530 on week long stays. Baffert Savings $550. Select homes only. Minimum 7 day stay required.

[Tim Harford]

Hello and welcome to the last in this series of more or less. We've been on a journey together, but this is our final climactic word on the numbers in the news and in life. And I'm Tim Harford. Right, what topics are epic enough to suit the occasion? Let me see. Did a Canadian politician misspeak when describing economic data? Are 1990s pop icons right, said Fred. Right about what? They said, can I run on a treadmill in a funny mask? I really want to scratch my nose. And is a sauna really 10 times as hot as whales? And in case you think we've all gone a bit silly, 25% of those items are more serious than you might imagine. But first, would you be better off working or claiming benefits? That depends on how much your job pays, of course. But there are lots of people who will tell you that it needs to pay a lot.

[Richard Varden]

You'd need a 71,000 pound pre tax household income to be as well off as a couple with children on benefits. Now, it seems that somebody who goes to work has to earn 71,000 pounds in order to match the benefits that are paid to a workless household who are signed off sick.

[Tim Harford]

With three kids, you now need to earn around 71,000 pounds before tax just to match what a jobless household with three children can receive in support. So where does this figure come from and is it a fair comparison? To help us understand, I am joined by our editor and once in a series, reporter Richard Varden. Hello, Richard.

[Richard Varden]

Hello, Tim.

[Tim Harford]

So where does the number come from?

[Richard Varden]

It comes from research done by the Centre for Social justice, which is the.

[Tim Harford]

Think tank set up by the former Conservative leader, Sir Ian Duncan Smith.

[Richard Varden]

Yes, they produced a report in December called the Benefits Budget as a response to the government's actual budget in November where the Chancellor, Rachel Reeves, got rid of the two child limit on universal credit. The Centre for Social justice wanted to highlight the dangers of perverse incentives within the benefit system.

[Tim Harford]

So where did they get this £71,000 figure from? Because it does seem like a lot. I mean we'd never pay producers on more or less that much.

[Richard Varden]

No, of course not. What would they even do with that amount of money? 71,000 pounds would put you near the top 10% of earnings and is almost twice the median full time earnings before tax, which is £39,000. So it's a lot of money. But there is more to this comparison than meets the eye.

[Tim Harford]

You don't say.

[Richard Varden]

First of all, the comparison is between what a whole family could get on benefits and, and the earnings of one individual. And of course 71,000 is not a take home amount. It reflects earnings before income tax and national insurance. That might be obvious to some. But less obviously, the Centre for Social justice also takes off a 9% student loan repayment and a 3% pension contribution. All of which implies a take home pay of just over £46,000.

[Tim Harford]

I guess not everyone has a student loan to repay. But deducting tax is reasonable enough. That's part of the point of the comparison after all. But why compare a family of five to one person?

[Richard Varden]

I put that to Jo Shallum, the policy director of the Centre for Social Justice.

[Tim Harford]

The reason why we opted for a single salary to kind of provide this comparison was to make clear the level of gross salary needed to reach the kind of income that can be achieved by an out of work household. In that context. Okay, so the person earning £71,000 takes home £46,000 and we're comparing that with a family on benefits. On the other side of the equation.

[Richard Varden]

Yes, according to the report, a couple on Universal credit with three children can get about £46,000 on benefits.

[Tim Harford]

Is that true?

[Richard Varden]

Well, benefits calculations are difficult and very individualised. It is very dependent on where you live, for instance, and the rental costs in your area. But now that the two child limit is gone, a three child family in London can get about £25,000 a year in benefits. Outside London it's about 22,000.

[Tim Harford]

That's a long way from £46,000.

[Richard Varden]

Yes, that's because of the benefits cap, a policy brought in by the then Chancellor George Osborne in 2013 to limit how much money a household can receive in benefits and therefore encourage people to seek work. It, in effect, limits how much money the Government will pay towards your housing costs.

[Tim Harford]

Does this benefit cap apply to everyone?

[Richard Varden]

No, there are a few exceptions, but the key one here is that if someone in the household is claiming a benefit for disability or serious illness, the benefit cap does not apply to the household. And the example family used by the Centre for Social justice is a family where one of the adults has an illness or disability that affects their ability to work.

[Tim Harford]

Right. And that can make a 20,000 pound difference?

[Richard Varden]

Yes, it can. The health element of universal credit is worth about 5,000 per year. Average personal independence payments are 7,500 a year and you could make up the rest with increased housing support because you weren't being subject to the benefits cap.

[Tim Harford]

So you can kind of make this equation balance in that situation. But does it make sense to compare two very different households?

[Richard Varden]

Well, people like benefits expert Gareth Morgan of the blog Benefits in the Future don't think it's helpful.

[Tom Coles]

The whole comparison is unfair, comparing a.

[Richard Varden]

Single person without disabilities against a family with children and rent and disabilities and all the costs that go along with them. But to Jo Shalom, it's important to recognise the incentives in the system and big rises in claims for conditions like anxiety and depression, which have more than doubled since 2019.

[Tim Harford]

If you are able to essentially double your benefit income by going through the gateway that says you are unable to work due to anxiety and or depression and you're entitled to PIP to support your independent living in that way. The additional costs that come with anxiety or depression say. I think we have to treat that as a very real incentive.

[Richard Varden]

Of course, others like Gareth Morgan disagree strongly.

[Tom Coles]

The incentive to be disabled is a ridiculous kind of concept in the first place. But those people who work in the advice sector will tell you unanimously how.

[Richard Varden]

Difficult it is to get these disability.

[Tom Coles]

Benefits and the tests and the hoops that you have to jump through, it is not something you can decide to claim. You have to prove your disability. And that's a very, very lengthy and onerous process.

[Tim Harford]

Well, as an economist, I believe incentives are always going to influence human behaviour, but the question is by how much?

[Richard Varden]

Yes, and we went looking for evidence and found some IFS research, that's the Institute for Fiscal Studies that found that lowering the benefit cap in 20 did induce more households to claim a benefit that would exempt the claimant from the benefit cap in the following year, for example a disability benefit. But the effect was not big, about a 2% increase in the numbers claiming when compared to those not affected by the benefit cap. That effect might now be bigger because of the cost of living crisis and the increased level of disability benefits, but with the current research base, we can't be sure of that.

[Tim Harford]

Thank you, Richard. Last week our inbox was flooded with request quests to investigate a dodgy claim made by a world leader from the stage at the World Economic Forum at Davos. I think we have a clip we can play now.

[Tom Coles]

Canada was amongst the first to hear.

[Tim Harford]

The wake up call leading us to.

[Tom Coles]

Fundamentally shift our strategic posture.

[Tim Harford]

Ah yes, I definitely assumed it was going to be Mark Carney, the Prime Minister of Canada. He was essentially arguing that we're seeing a new era of geopolitics we one in which Western powers can no longer count on the United States as a reliable security and economic partner. Which sounds worrying. But While taking questions, Mr. Carney had a reassuring statistic.

[Dr. Danny Muniz]

I'll give you again, I'll appeal, since.

[Tom Coles]

It'S in the headlines, to the Nordics.

[Tim Harford]

Nordics plus Canada, it's 20% of global GDP. The Nordics and Canada making up 20% of global GDP Really? Well, when you look at data from the International monetary fund for 2025, Mr. Carney is way off. Canada is 2% of global GDP and if you add on the Nordic nations of Iceland, Norway, Sweden, Finland and Denmark, you get to about 3.5% of global GDP. So what's going on? Well, it turns out Carney meant to say Canada plus the eu. So that doesn't include Norway and Iceland, but it does add in the likes of Germany, France, Italy, Spain and many other rich countries and that would indeed bring us to 20% of global GDP. And just for context, the US economy accounts for about 25% of global GDP. You're listening to more or less. Social media is the town crier of our times, the way many people get their news. But the problem is that just as anyone can raise their voice and shout in a town square, although I wouldn't recommend it, people start giving you very funny looks. Anybody can post something online and claim it is definitely, irrefutably fact. Share it like it, repost it, repeat. Alas, people are often wrong and these alleged facts are often distorted. We came across one of these very online claims the other day, and here to tell me more is Lizzie Mcneil Hello, Lizzie.

[Lizzie McNeil]

Hi, Tim. Well, I can't believe I'm about to say this, but I've been fact checking. Right Said Fred.

[Tim Harford]

Right Said fred, as in 90s pop band. Right, Said Fred.

[Lizzie McNeil]

Yeah. The I'm too sexy for my shirt. One hit wonder, guys.

[Tim Harford]

One hit. I don't think so, Lizzie. Deeply Dippy was a musical tour de force. Why are we talking about Right Said Fred on more or less?

[Lizzie McNeil]

Well, I've been looking at something. They tweeted the Home Secretary, Shabana Mahmood, after a recent arson attack on a mosque in Peace Haven was where masked men were seen purposefully setting fire to the building. She said it was deeply concerning. The Freds, as they're known on X.

[Tim Harford]

Replied, 9,000 crimes against churches in the UK in the last three years. I don't recall you finding those deeply concerning. OK, but the big question is, is what Rightshead Fred said, right?

[Lizzie McNeil]

No, what the unspecified Rightshead Fred said is not right. Now, to be fair to them, their tweet contained a link to an article by the National Churches Trust, which led with the nine figure. And this number also appeared in an article by the Telegraph.

[Tim Harford]

Ah, so the media is complicit too.

[Lizzie McNeil]

Yes, and the statistic comes from a report by the Countryside alliance, which is a group that campaigns about issues affecting rural communities. And they put in a Freedom of Information request to all of the police forces in the uk. Through this, they found that the police have recorded over 9,000 crimes that took place in churches.

[Tim Harford]

Okay, but surely that sounds like the claim is true.

[Lizzie McNeil]

Sounds like it, yeah. It's understandable to see the confusion, but it doesn't mean what you think it means. You see, a lot of police forces include all places of worship within their count. So mosques, Buddhist temples and synagogues will be included in these numbers.

[Tim Harford]

So it doesn't just mean Christian churches.

[Lizzie McNeil]

That's right. And the Telegraph got round this by talking about churches and places of worship, but all of their case studies were Christian churches, so it's a tad misleading.

[Tim Harford]

Yeah.

[Lizzie McNeil]

Now, churches do vastly outnumber other religious buildings. In the UK, there are about 38,000 churches compared to around 1,800 mosques, 450 synagogues and 200 Buddhist temples. So from a numbers perspective, you would expect churches to have more attacks as there are just more of them and.

[Tim Harford]

People use the phrase crimes against churches. Were these crimes attacks?

[Lizzie McNeil]

No. So the majority of incidents were thefts and burglaries.

[Tim Harford]

So stealing artwork, antique lecterns, that sort of thing?

[Lizzie McNeil]

Yeah. And lead from gutters and roofs. There were over 3,000 cases of criminal damage and 2,102 cases of violence.

[Tim Harford]

So not all of these attacks are carried out as a statement against religion.

[Lizzie McNeil]

No. And if you take West Yorkshire as an example, their stats include nearly 100 incidents of stalking and harassment, a case of drug trafficking and 11 incidents of rape.

[Tim Harford]

Yeah. Drug trafficking might be a crime committed in or near a church, but it would be weird to describe it as a crime against a church.

[Lizzie McNeil]

Yeah. Now, some of these crimes could reasonably be described as crimes against churches or mosques or synagogues, but many of them are better described as crimes on the property. Even when a church or its contents is the target, this statistic is about where the crimes took place, not why. Churches may be the target of a religiously motivated crime, but they're also vulnerable to theft or vandalism because they're often in secluded areas, open and unmanned.

[Tim Harford]

So, looking through the responses to this tweet and other conversations being had, there is a perception that Christians are being attacked for their beliefs and that church fires are a symptom of that and that no one's acknowledging it.

[Lizzie McNeil]

Yeah, that is what they're saying. And the Countryside alliance data doesn't tell us anything about whether that's true. We could look instead at religious hate crime in the UK. According to the Home Office, in the year ending March 2025, the police recorded just over 7,000 religiously motivated hate crimes. 45% of those crimes were against Muslims and 29% against Jews. Only 5% were against Christians and the rest 6% Sikh, 3% Hindu, and the rest were either other unrecorded or unknown.

[Tim Harford]

So the majority of religious hate crimes are being committed against people who are Muslim or Jewish.

[Lizzie McNeil]

Yeah.

[Tim Harford]

Now, this argument kicked off because of a fire at a mosque, and the inference is that violence and arson is also happening to Christian places of worship. So is that true?

[Lizzie McNeil]

Well, it has definitely happened. I spoke to Ecclesiastical, an insurance company that specialises in heritage buildings. They were set up 180 years ago after a spate of church fires. And they told me that in the last 10 years, their clients have seen 161 fires that were deemed arson and 267 which were accidental.

[Tim Harford]

Do we know who's setting these fires?

[Lizzie McNeil]

So neither the Countryside alliance nor Ecclesiastical Insurance can tell us that. I've been looking into newspaper reports, so that's basically just a large collection of anecdotes. But those newspapers generally mention teenagers, people with mental health problems, people who are under the influence or. And this is very much a church specific hazard. People knocking over votive candles. This was not an exhaustive search, but I couldn't find any evidence of Christian buildings being systematically burnt down by people with anti Christian intent. Christians are at risk of violence in some countries around the world, but thankfully there is currently very little evidence that Christians in the UK face any real threat of violence.

[Tim Harford]

Thank you, Lizzy.

[Lizzie McNeil]

If you're an H Vac technician and a call comes in, Grainger knows that you need a partner that helps you find the right product fast and hassle free. And you know that when the first problem of the day is a clanking blower motor, there's no need to break a sweat. With Grainger's easy to use website and product details, you're confident you'll soon have everything humming right along. Call 1-800-GRAINGER clickgrainger.com or just stop by Grainger for the ones who get it done.

[Tom Coles]

If you're the purchasing manager at a manufacturing plant, you know having a trusted partner makes all the difference. That's why, hands down, you count on Grainger for auto reordering. With on time restocks, your team will have the cut resistant gloves they need at the start of their shift and you can end your day knowing they've got safety well in hand. Call 1-800-GRAINGER click granger.com or just stop by Grainger for the ones who get it done.

[Tim Harford]

Now the moment we've all been waiting for. The culmination of weeks of planning and hardship. Some of you said a small team like ours didn't need a specialist sauna correspondent. Oh, how wrong you were. And it's all thanks to Mike, Ricky and Bob. These brave and loyal listeners got in touch to ask us to look into a number they heard on Radio 4 programme, Inside Health. A number related to that most glorious of things, a sauna. Comparing the temperature outside in Wales to the temperature inside said sauna physiologist Professor Damian Baillie made the following throwaway comment talking to presenter James Gallagher. Just so we're clear, I'm cold already. Because it's a wintry day in Wales. Eight degrees at the moment, so 80 degrees centigrade in there, right?

[Richard Varden]

Ten times as warm.

[Tim Harford]

Ten times as warm, eh? Our listener smelt a sweaty rat. And who else to set the record straight but our very own sauna correspondent, Tom Coles.

[Tom Coles]

It's happening, Tim.

[Tim Harford]

I know.

[Tom Coles]

We can finally turn down the thermostat in this studio.

[Tim Harford]

Yeah, and you can put some clothes on.

[Tom Coles]

You'll be lucky.

[Tim Harford]

Okay, let's do this. Professor Bailey is no Doubt a very fine physiologist. But he did say a silly thing, didn't he?

[Tom Coles]

He did, yeah. I enlisted the help of Professor Kelly Morrison, Head of Physics at Loughborough University, to explain what's gone wrong here.

[Lizzie McNeil]

I think for the maths, the maths is fine, so 80 is 10 times 8. The problem is what you mean by warmer. So because for the scale that you've chosen to use there, zero degrees Celsius in this instance is not as cold as it can get. So it's at that point that the math starts to break down.

[Tom Coles]

The Celsius temperature scale is relative. 0 is just the temperature that water freezes. It's not the absence of all heat. You can see this problem if you convert to Fahrenheit instead. That's the one they use in the us and weirdly in British newspapers on a hot summer's day, because saying the temperature is in the 80s is just more exciting than saying the temperature is 27 degrees. The numbers are bigger because the scale is relative to something else. It runs between the freezing point of brine and the temperature of the human body. So although it's quite satisfying for the Phew. What a Scorcher headline. It makes less sense when you realise the freezing point of fresh water on the Fahrenheit scale is 32 degrees. For our Welsh sauna, it's 46 degrees Fahrenheit outside in wintery Wales and 176 degrees in the sauna, which is only four times warmer.

[Tim Harford]

Right, so these relative scales make all the multiples meaningless. I mean, how many times warmer is 1 degree compared to minus 1? It doesn't make any sense. So come on then, give us the solution.

[Tom Coles]

Right, so in order to do the maths properly, you have to work out the temperature on a different scale, one where zero doesn't just mean frozen water or frozen brine, but one which starts at absolute zero. No heat at all, nada thermal energy. And the temperature scale that does this is the Kelvin scale. Zero degrees Kelvin is minus 273.15 degrees Celsius.

[Lizzie McNeil]

If you wanted to take your 8 and 80 degrees Celsius example, you could go from 8 degrees Celsius to Kelvin by adding 273. So that would give you 281.15. If you cared about the decimal points Kelvin for 80 degrees Celsius, that would be 353.15 Kelvin. And the ratio of the two of these is actually 1.256. That's about 1.26 times warmer.

[Tim Harford]

So if you are respecting physics, then a auna is only 1.26 times warmer than outdoor Wales in winter. Got it. Just out of curiosity, Tom, how hot would the sauna need to be for it to be actually 10 times warmer?

[Tom Coles]

About 2,500 degrees C. That's 1,000 degrees hotter than lava. Or around the temperature of an industrial furnace which can melt steel like butter.

[Tim Harford]

I don't think I'd go into that sauna. Okay, Tom, your worker's sauna correspondent is done. Please, if you would at least cover yourself with a towel. As you wish. In 2017, Eliud Kipchoge, one of the greatest athletes the world has ever seen, tried to redefine the capability of the human body and break one of the most imposing boundaries of human achievement. He tried to run the marathon distance of 26.2 miles in under two hours in a project called Breaking2. Kipchoge was agonisingly close. He was just 25 seconds away from history. Two years later, he tried again and succeeded. So where does humanity go from here? Should we weep for we have no more worlds to conquer? Well, human achievement's all relative, you see. I've got a charity place in the London Marathon in April. It's my first. And I've got my eye on a target that is surely just as impressive. I'm Tim Harford and this is breaking four.

[Richard Varden]

Breaking four, yes.

[Tim Harford]

Can a Radio 4 presenter, his legs atrophied by nearly two decades, sitting at a studio desk, run a marathon in under four hours? To find out if this mind boggling target is even feasible, I'm going to need some good hard data. Luckily, Dr. Danny Muniz, a senior lecturer in exercise physiology at the University of Hertfordshire, invited me to their shiny, exciting sports lab. Picture oxygen masks, heart rate monitors and fancy treadmills, all set up to capture my vital statistics. And there are two metrics that matter particularly. One is called VO2max and the other the lactate threshold. Let's start with VO2max. Here's Danny.

[Dr. Danny Muniz]

V stands for the volume. O2 is oxygen. So it's the volume of oxygen that you can take in but also consume. And max just stands for the maximum, the highest amount of oxygen that you can use, which is going to happen during intense exercise. VO2 max essentially sets the ceiling of your aerobic metabolism.

[Tim Harford]

VO2 max is the amount of oxygen you can take in when you're pushing yourself as hard as you can go over a short distance. Of course, that's not really an option in a marathon. So the key question is, how close to your top capacity can you run without becoming so fatigued that you slow down? This is where the lactate threshold comes in.

[Dr. Danny Muniz]

When we are running a marathon, we are not sprinting, we are not running as fast as we can. We need to run at a pace that is sustainable for the duration of the event. So those lactate threshold, or physiological thresholds, essentially determine how close you can go to your VO2 max. What we tend to see is that trained individuals, elite athletes, not only have a much higher VO2 max, but they can also sustain a much higher percentage of their VO2 max. So they will be exercising at 95, 98% of their VO2 max for a long time.

[Tim Harford]

Lactate is a substance that's produced in our muscles when we exercise at intense levels. It's a signal of fatigue, a sign that we're not going to be able to sustain that pace. The pace at which I can run the marathon is going to be strongly influenced by a combination of these two factors. VO2 max, my top capacity, and my lactate threshold. The level of that capacity I can sustain without becoming too fatigued. I'm going to be running on this treadmill. How are you going to measure these physiological numbers?

[Dr. Danny Muniz]

So we will be starting at a really low pace, running for a few minutes, three, four minutes. And then we take a small blood sample from the finger and we measure how much lactate is in blood. And we do that at increasing speeds, again going up slowly, until we see that lactate starts to increase above resting values. VO2 max. Obviously, we said that VO2 max is the maximum amount of oxygen that we can consume. So VO2 max will require your team to exercise maximally, which means exercising for as long as you can. As the speed goes up every minute, I'm absolutely terrified.

[Tim Harford]

Is that wrong?

[Dr. Danny Muniz]

Well, it's normal, but it's. I said this to you before, Tim. It's not as bad as it sounds. Okay? Okay.

[Tim Harford]

Well, I'll be the judge of that.

[Richard Varden]

Breaking four.

[Dr. Danny Muniz]

So now, Tim, this is when we need to check that this is tight enough.

[Tim Harford]

I don't have a lot of experience with wearing tight.

[Dr. Danny Muniz]

Basically, just go with how it feels.

[Tim Harford]

Regardless of the rumors, I've always wanted to say this, this time for a training montage.

[Dr. Danny Muniz]

So we're going to go at 8 kilometers per hour. So whenever you're ready, you can go for it. And we start him now.

[Tim Harford]

Now I really want to scratch my nose.

[Dr. Danny Muniz]

10 seconds and we go up again. Big push.

[Tom Coles]

Okay.

[Dr. Danny Muniz]

Really good work.

[Tim Harford]

I just got Pricked again.

[Dr. Danny Muniz]

Come on, team. Big effort now, okay? Big push now.

[Tim Harford]

Yes, sir. Come on. 15k an hour. It's 20 minutes. 12k an hour, which I just did, is 25 minutes. This must be about 23 minutes. Don't make me come over there. I don't think a wiper is going to do it.

[Dr. Danny Muniz]

Three, two, one. And stop. There you go. Well done.

[Tim Harford]

It's gonna get harder. I know it's gonna get harder. How's it doing, Danny?

[Richard Varden]

He's doing really well.

[Dr. Danny Muniz]

He's got about a minute left, maybe.

[Tim Harford]

Come on, team.

[Dr. Danny Muniz]

Keep it going. Keep going, keep going, Keep going, keep going.

[Lizzie McNeil]

Let's go, let's go.

[Dr. Danny Muniz]

Excellent work.

[Tim Harford]

Really, really good.

[Lizzie McNeil]

Great stuff.

[Dr. Danny Muniz]

A fantastic effort.

[Tim Harford]

Tired. There's a certain point where the brain is just. No, this is very bad.

[Tom Coles]

I'm done.

[Tim Harford]

That's it.

[Tom Coles]

Too much.

[Dr. Danny Muniz]

You want to just walk for a couple of minutes, Nice and easy?

[Tom Coles]

Sure.

[Tim Harford]

Finished the treadmill test about 10, 15 minutes ago. I've got my breath back. So, Danny, what did you find and what does that tell us?

[Dr. Danny Muniz]

We've got some numbers here, so we did estimate your likely threshold to happen at around 11 km per hour. I would suggest that that's a speed that you can probably maintain based on the data for the marathon. That corresponds to a Marathon time of 3 hours and 50 minutes.

[Tim Harford]

350. So I've got 10 minutes spare. Yes. Which I will need. But I sense a hesitation in your voice.

[Dr. Danny Muniz]

It's very difficult to predict the exact marathon and so many things can affect the result on the day from your nutrition, how well you sleep, how you feel in the day, the weather. But based on the numbers, I think that you can go under four hours. If you keep training.

[Tim Harford]

It'S going to be a long three months. My thanks to Dr. Danny Munith and his team at the Institute of Sport at the University of Hertfordshire. This is the end of our Radio 4 series, but don't despair. You can catch us all year round on our weekly Saturday podcast. We'll be back on Radio 4 in May, by which time, hopefully, I'll have done that marathon. Please send in your questions and your comments to more or lessbc.co.uk and until next time, goodbye. More or Less was presented by me, Tim Harford. The producer and sauna correspondent was Tom Coles with Nathan Gower and Lizzie McNeil. The production coordinator was Brenda Brown. The program was recorded and mixed by Gareth Jones. The breaking for Sting was created by James Beard. The programme was edited by Richard Varden. Do the Wonder products that you see on your social media and supermarket shelves really deliver on their bold claims. Dehumidifiers, standing desks, nail polish from supplements claiming to boost your mind and body. I've seen so many claims about creatine to fake tans promising a safe streak free glow.

[Lizzie McNeil]

I really like it.

[Tim Harford]

I'm Greg foot and my BBC Radio 4 show sliced bread is back. To separate more science fact from marketing fiction. I would tend to lean towards it being a positive. All our suggestions come from your emails or voice notes, even if you're a bit under the weather.

[Lizzie McNeil]

Hello Greg, I want to know about cough mixture.

[Tim Harford]

I'm finding out the answers in my new series of Sliced Bread, available first on BBC Sounds.

[Lizzie McNeil]

If journalism is the first draft of history, what happens if that draft is flawed? In 1999, four Russian apartment buildings were bombed, hundreds killed. But even now we still don't know for sure who did it. It's a mystery that sparked chilling theories. I'm Helena Merriman and in a new BBC series, I'm talking to the reporters who first covered this story. What did they miss the first time? The History Bureau Putin and the apartment bombs. Listen on BBC.com or wherever you get your podcasts.