In an episode from the fifth season of Grey’s Anatomy, Miranda Bailey, played by Chandra Wilson, orchestrates an ambitious 12-person paired kidney transplant surgery. Six living donors each give one of their kidneys to a stranger and six recipients receive one in return. While the show, which is still on the air almost 18 years later, often blurs the line between fact and fiction, this particular form of kidney transplant, called a paired kidney exchange chain, has become increasingly common in the United States. 

Rewatching the show (sue me), I wondered if such donation chains are viable for other organs (or tissues) that can be donated by living donors. The liver, the only organ that can regenerate itself over a span of six to eight weeks, struck me as a particularly notable candidate. As with any surgical procedure, donating part of one carries some risk, but in most cases a person can safely continue to lead a healthy life afterward. And the need for them far exceeds what deceased donors can provide. Right now, there are more than 100,000 people on the waitlist for organs.

Over the past decade, using organs from living donors to meet organ demand has emerged as a particularly effective strategy. 

In 2025, around 24 percent of kidney transplants used living donors, according to preliminary data from the Organ Procurement and Transplantation Network (OPTN), the public-private partnership responsible for maintaining equitable organ allocation policies. Infrastructural advances, such as the establishment of the National Kidney Registry (NKR) in 2007, have helped reduce wait time for kidneys, provided financial support for donors, and facilitated close to 11,000 paired exchange kidney transplants. In a paired exchange, two patients who each have a willing but incompatible organ donor simply swap donors — my donor gives to you, yours gives to me. Ultimately, both of us receive a kidney that our bodies are more likely to accept.

By contrast, only about 6 percent of the roughly 12,000 liver transplants performed in the US last year used living donors. 

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This is not the case around the world. Countries that face a shortage of deceased liver donors rely almost entirely on livers from living donors. In 2022, India placed third, behind the US and China, in terms of the overall number of liver transplants but performed the most living donor liver transplants in the world — 81 percent of all liver transplants there were from living donors. 

While the number of living donor liver transplants has trended generally upwards over the last decade, the success of living kidney donation suggests that a similar path might be possible for the liver here, too. The US already has what it takes — a potentially lifesaving supply of living donor livers — but it lacks the infrastructure to turn willing liver donors into transplants at scale. That may actually be starting to change, as surgeons around the country are working to build out the interconnected systems and standards that they need to do their work. 

Why parting with part of your liver makes all the difference

Organ access is a multifaceted problem in the US. While the country has largely drawn on organs from deceased donors to meet organ demand, living donors grow more important by the day. A patient’s position on the liver waitlist is determined by their Model for End-Stage Liver Disease (MELD) score. It is a number between 6 and 40 that helps doctors decide how soon the patient needs a new liver. Sometimes, if a patient gets too sick, surgery is unsafe, or if they have problems with substance abuse, they are no longer transplant eligible. 

Living liver donors are especially important because transplantation is the only treatment for end-stage liver failure — there are no alternative options. For patients in end-stage kidney failure, a dialysis machine can artificially take over the function of a kidney, and temporarily help manage their condition as they wait for an organ match. But a person experiencing liver failure has no such interventions available to them. They continue to get sicker and sicker the longer they hold on. Around one in five liver transplant candidates die waiting for a new liver. 

Living liver donors go through extensive health screens to ensure that their liver quality is good for transplant. Research indicates that the resulting donated organs lead to equal or better overall survival, compared to deceased donors. A transplant surgery with a living donor can also be scheduled, unlike those that rely on deceased donors, which are based on when an organ becomes available. This means that living donor livers usually don’t have to be transported long distances (since the donor can travel to the recipient before the surgery) and the amount of time the liver is without blood and has to be artificially preserved is very short — an important metric in donor organ viability.  

Earlier this year, doctors at the University Health Transplant Institute in San Antonio performed a seven-pair living donor liver exchange, the largest such chain in the country. Over a period of six weeks, the liver transplant surgeons matched seven people willing to donate a portion of their livers to seven other recipients who needed the life-saving transplant. Like most of these paired exchange chains, the logistically complicated process was set into motion by a non-directed living liver donor — someone who was willing to donate a liver to a stranger.. Other donors in the chain, by contrast, had a specific person they hoped to donate to with whom they weren’t immediately compatible.

Blood type and liver size are two main considerations when doctors match donor livers to recipients. Individuals with Type O blood usually have the longest wait times for donor livers because they can only receive them from others with the same blood type. In other cases, the size or anatomy of the donor liver can be incompatible with the recipient. 

An estimated 2 percent to 17 percent of living liver donors are rejected from donating to their intended recipient because of blood type, size, or anatomical incompatibility of the system of bile ducts. A major benefit of living donation is that if someone wants to donate their liver to a specific person, but they are not a match, both the donor and the recipient can participate in liver paired exchange programs — two or more incompatible donor-recipient pairs can swap donors. 

“For each living donor transplant that you do, you are actually doing two transplants,” said Dr. Jonathan Cullen, a liver transplant surgeon who participated in the liver paired exchange at University Health Transplant Institute. 

By that, he means that any time a living donor provides a liver, they also free up an organ on the deceased donor list for someone who might need it more. If we had more living donors, we could save even more lives — especially of those lower on the waitlist — but we need to overcome a few barriers first.  

Growing the donor pool isn’t enough

If living donor liver transplants have so much potential, why are they still so underutilised? The answer depends on whom you ask. 

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When medical researchers from Weill Cornell Medicine and the University of Colorado Denver surveyed 99 liver transplant programs in the country, surgical and medical directors identified different barriers to living donor liver transplants based on whether their program already performed such procedures. Out of the 83 that responded to the survey, around 60 percent of the centers performed living donor liver transplants. Notably, 34 percent of the responses from centers that don’t perform living donor liver transplants cited institutional issues such as lack of financial support or a lack of surgeons trained in living donor procedures, which are meaningfully more complex than kidney transplants. 

“Center expertise is very important when it comes to living donations. You want to obviously have your operation done at a center that’s done a lot of these,” said Alyson Kaplan, a transplant hepatologist at Tufts Medical Center and lead author of the study. “Not every transplant center has a living donor program — and so, as you can imagine, not every transplant fellow is going to get exposed to living donation.”

The surgeries are also resource intensive, especially when they are paired exchanges. In the seven-paired liver exchange at the University Health Transplant Institute in San Antonio, two attending faculty surgeons were required for every donor operation and two to four of the same for the recipient surgeries, according to Cullen. 

Plus, many of the steps in the liver transplant process are not standardised. There doesn’t yet exist a central organ matching organization for living donor livers along the lines of the National Kidney Registry, and there are no formalized protocols for how living livers are chosen and allocated, or how potential donors are screened. These procedures can differ from center to center, which can complicate attempts to collaborate or share resources. 

With living kidney donation, by contrast, the NKR partners with transplant centers and helps them standardize their processes, said Kaplan. The majority of certified kidney transplant centers in the US perform living donor transplants. That combination of shared standards and widely available surgeons makes it easier for one team to procure the organ from the donor and another to perform the recipient transplant, even if the donor-recipient pair is geographically separated.

Donor kidneys can be matched efficiently to recipients thanks in part to the work of American economist Alvin Roth, who won the Nobel prize in 2012 for the theory of stable allocations and practice of market design. His work increased the efficiency and speed of several matching processes, including paired donor-recipient kidney allocations and residency matching for medical students.

“The kind of algorithmic help that people like me can give is very, very similar,” said Roth on whether his work can be applied to liver transplants too. “Almost the same software works. What changes is the compatibility matrix.” 

For now, individual institutions that perform paired exchange living donor liver transplants usually do the matching in-house, manually comparing donor-recipient profiles and screening for risks. Kaplan’s study found that 92 percent of living donor liver transplant programs that had never performed this surgery would consider conducting it. 

But without enough funding, Kaplan said, a big infrastructural boost like this one is hard to execute on a national scale.  

Who protects the donors?

Both infrastructure and funding are necessary to scale living donor liver transplants nationwide. However, without a large pool of willing donors, these resources alone are insufficient. 

But building a bigger pool of donors is difficult, partly because becoming a living donor is difficult. 

Living liver donors often have to travel to their recipients, pay for associated lodging, and take weeks off work for both the operation and the recovery. The cost of donor screenings and transplant surgeries is usually taken on by the recipient’s insurance, but many programs recommend that the donor plan for any additional expenses, including follow-up appointments, continued medications, and caregiver expenses. 

Nonprofits like the National Living Donor Assistance Center (NLDAC) can help living liver donors subsidize costs, but only if the recipient’s household income is 350 percent below the federal poverty guidelines. For a household of two in the 48 contiguous states, that is an annual income of $75,740. Some states also provide income protection for living donors who have to take time off work, but others don’t. In the absence of more universal protections and other forms of support, building the donor pool may be a bottleneck for also building out broader living liver donation infrastructure. 

“Donors should never have a penalty to be a donor. They are definitely the national heroes, and they should have all the protections that they deserve. Nobody should have to go into debt to be a donor, for goodness sakes,” said Dr. Julie Heimbach, director of the Transplant Center at Mayo Clinic. The center performed its first paired living liver transplant in 2024. 

Moving the needle — and the organs

One major roadblock in establishing national collaborative programs for living liver transplants comes down to what’s called the organ’s cold ischemia time. This is the time between organ procurement and transplantation, during which the organ is kept cool, and no blood flows through it. 

Kidneys can hold on for longer than livers, which means that they can be transported across significant distances without the quality of the organ deteriorating. With livers, it becomes all the more important to quickly procure and transplant them because they can’t stay on ice too long. 

This limits how far a liver can be transported. A liver from a living donor across the country might not be viable by the time it reaches the recipient and is transplanted — and even if it could be, a transplant surgeon may not be willing to take a risk on it. 

In 2023, when United Network for Organ Sharing (UNOS) Labs, the innovation arm of the private non-profit that manages the organ transplant system in the US, attempted to pilot a nationwide liver paired donation program, this proved to be one of the biggest stumbling blocks. The program was anonymously funded by a private benefactor and involved 15 transplant centers across the US. However, the program shut down less than a year later. Even though one recipient-donor pair was enrolled for a liver paired exchange, the transplant surgeries were ultimately not executed. Many of those I spoke to for this story — transplant surgeons from centers that participated in the pilot — said that the funding dried up before the challenges of a collaborative liver paired exchange surgery could be resolved. 

“One of our key learnings was that moving donors, rather than organs, created major barriers. Donors are connected to their care teams and support systems, and asking someone to relocate for surgery and recovery is difficult,” Dr. Benjamin Samstein, the lead on this pilot and chief of liver transplantation at NewYork-Presbyterian and Weill Cornell Medicine, said in an email. 

He believes that there is still potential to revisit the concept, this time with a focus on moving organs instead of donors and exploring regional hub models. 

As Vox reported earlier this year, some transplant surgeons now rely on a new method of organ preservation where an external pump is used to circulate oxygenated blood or other fluids through an organ after it is procured from the donor. This keeps the donated organ viable for much longer and has been shown to outperform standard “icebox” preservation where the organs are cooled on ice. That could, transplant surgeons say, spare living donors the obligation of traveling to their recipients. 

While this technology, which is also revolutionizing the use of organs from deceased donors, may eventually reduce the pressure to recruit living donors, Dr. Jason Hawksworth, Director of the Robotic Liver Surgery Program at New York-Presbyterian/Columbia University Irving Medical Center, doesn’t think that it will diminish their importance. 

The MELD score used to prioritize patients on the liver transplant waiting list does not capture all clinical symptoms. Patients with low MELD scores can also face life-threatening complications of liver failure such as abnormal buildup of fluid in the abdomen. Such patients benefit most from living liver donations because they don’t have access to machine-perfusable organs usually reserved for those higher on the waitlist, said Hawksworth. 

To meet at least some of the need, the American Liver Foundation launched the first-ever non-directed living liver donation network to connect those who would like to donate to a stranger to transplant centers who can access this network free of charge. While the network has already saved two lives as of June this year, it remains to be seen if the program will run into the same issues as its more ambitious UNOS counterpart. 

Ultimately, optimizing the field of organ procurement and transplant is both a medical problem and an infrastructural one. OPTN data shows that 95 percent of those on the organ waitlist are in need of a kidney or a liver — the two organs a living person could give. Kidney donation has already been transformed by a combination of smarter algorithms, better standards, and trained surgeons. While living liver donation presents more challenges, there’s a real possibility to revolutionize the process and save lives.

“If these particular barriers were not an issue,” Kaplan said. “I think everyone would be interested in it.”

Popular conceptions of farming tend to associate good animal welfare with environmental virtue — an intuition so strong that researchers have described a “humane halo” effect, where consumers who are told that a farm uses better environmental practices also assume that it treats its animals well. So it might come as a surprise to hear that one way of making eggs better for the planet might be to make life a whole lot worse for the chickens who lay them. 

This, anyway, was the apparent upshot of a controversial and widely discussed new paper on the environmental impacts of different egg farming systems. Cage-free, free-range, and organic eggs, it found, are higher in greenhouse gas emissions, land use, and other environmental harms than confining hens in tiny cages. 

Consider the implications of that result: At any given moment, more than 8 billion egg-laying hens inhabit farms across the planet. The vast majority of them live in wire cages that severely restrict basic movements like spreading their wings and turning around. They barely have room to move around at all, which can induce depression-like states. “Hens represent the largest population of terrestrial animals housed in cages throughout their lives,” as the new study puts it. 

In animal agriculture, there is an unfortunate and well-known trade-off between environmental impact and animal welfare because, all else equal, animals who are allotted a bit more room to roam and express natural behaviors take up more resources. (Of course, that trade-off disappears if we simply replace animal products with plants.) That is why almost all animal products these days are produced on factory farms in the first place — factory farming doesn’t exist merely to be evil, but rather because it’s efficient, and the cost of efficiency is borne by the animals. 

Although the new paper may overstate the magnitude of this tension for eggs (more on that in a moment), it’s directionally right: Hens who are able to walk, flap their wings, fly, and perch will eat more calories than those trapped in cages, so their environmental footprint is generally going to be greater. 

But here is the thing: That doesn’t really matter. 

Eggs aren’t generally singled out by climate experts as a major decarbonization priority. Although animal agriculture is a significant contributor to climate change, making up about 12 to 20 percent of global greenhouse gas emissions, eggs are responsible for a very small share of that — about 3 percent of all livestock emissions. As a back-of-the-envelope estimate, that is, on the high end, roughly 0.6 percent of all emissions. I think we can spend a tiny bit of our carbon budget to spare billions of animals a life of near-total immobilization. 

But it’s weird to think about this as an environmental question at all. Chickens aren’t merely resource burdens — they’re inhabitants of our planet who deserve to share in the benefits of a more sustainable future, too. And the most salient feature of caging hens is not that it’s marginally more environmentally optimized, but simply that it’s wrong.

Caging humans would reduce our emissions, too, as Lewis Bollard, managing director for farm animal welfare at the philanthropy Coefficient Giving, grimly remarked about the paper. No one would ever suggest doing that, because a lower carbon footprint cannot turn a morally indefensible practice into a good idea. 

Is there really a climate case for cages?

Over the last few decades, animal welfare advocates have sought to bring hens some relief from extreme confinement, passing laws that ban cages and persuading grocery stores, restaurant chains, and other food purveyors to source cage-free eggs. (When I was in college, I worked on one of these campaigns, helping convince my school to serve exclusively cage-free eggs in its dining halls.) Nearly half of egg-laying hens in the US and two-thirds of those in the European Union are now raised cage-free, and four-fifths of eggs in the UK come from cage-free hens. But cages still prevail in some high-income countries, including Canada, Japan, South Korea, and Israel.

This advocacy work has spread to low- and middle-income countries, too, where most of the world’s hens live and overwhelmingly remain caged. Cage-free eggs are still far from cruelty-free, but they likely represent a meaningful welfare improvement. 

The new paper, published in Royal Society Open Science late last month, used data from the UK to compare the impacts of five different egg farm types: battery cages, slightly larger “enriched” cages, cage-free indoor barns, free range, and organic free range. Fully indoor cage-free systems are by far the most widespread form of cage-free farming in the US and elsewhere in the world, and they’re generally the default large-scale system used in transitions away from cages, so they are our primary focus here. 

The researchers compared egg farming systems on four environmental metrics: greenhouse gas emissions, nutrient pollution in waterways, soil acidification, and land use. The lowest impacts across all four categories were found in farms that used enriched cages, which are one-third to two-thirds larger than the standard battery cages that dominate the global egg industry  — the latter had the second-lowest impacts. (From an animal welfare perspective, enriched cages are akin to slightly loosening a straitjacket — only a grotesque starting point could make them look generous. They afford each hen a living space not much larger than a sheet of printer paper, well below the area typically needed to perform basic behaviors like flapping her wings and grooming herself.) 

To produce the same number of eggs, the study found, cage-free barns emit about 56.1 percent more greenhouse gases than enriched cages and 33.8 percent more than battery cages. 

The authors have said that the paper isn’t meant to advocate keeping hens in cages, but rather to inform decision-making about cage-free transitions with accurate estimates of the environmental trade-offs involved. The findings have been presented as relevant to food sustainability discussions globally, more broadly than just in the UK. And, Bollard pointed out to me, the paper has come out as the EU and UK weigh fully banning caged egg production, raising the possibility that its findings could influence those policy fights. 

“We recognise that the transition toward cage-free production is important for improving animal welfare,” lead author Oliver Martinić, who recently earned a PhD in animal and food science from the University of Padova, told me in an email. “The long-term goal should be to develop cage-free systems that combine high standards of animal welfare with improved production efficiency and lower environmental impacts. If we can achieve that, the environmental gap between cage-free and caged systems could be substantially reduced.” 

It’s certainly important to have an accurate accounting of the environmental impacts of all types of farming. Whether or not the paper achieves that has been a subject of some debate. To make their calculations, the authors drew from data published more than a decade ago, but cage-free egg production has since become more environmentally efficient. 

“The paper is methodologically flawed,” Ilias Kyriazakis, a professor at Queen’s University Belfast and senior author of the earlier research whose data the new paper relies on, wrote in a critique of the study. “It relies on our data from 2010 to construct scenarios intended to represent national conditions in 2023, without adequately accounting for the substantial changes that have occurred over that period.” As a result, he added, “the authors are likely to have overestimated some of the impacts” of the non-cage production systems.

The authors have said that they used that dataset because it’s the most comprehensive one available, and Martinić told me that “production systems have continued to evolve, so newer data would be valuable for assessing the extent to which the gap [between caged and cage-free eggs] has changed.” Other peer-reviewed research, meanwhile, has found a smaller gap in climate impacts between caged and cage-free eggs. Even the makeup of hens’ feed may matter more for the climate impact of eggs than whether they’re kept in caged or cage-free housing: One US study found that cage-free farms were lower in emissions and other impacts because they used more environmentally efficient feed.

In the end, these differences may not be that big of a deal because the climate impact of eggs is small in the grand scheme of animal agriculture. Using the new study’s own emissions estimates for eggs, a single beef burger emits as much as about five dozen cage-free eggs.

“There’s no doubt that eggs are the least environmentally impacting animal product per unit,” Kyriazakis told me. So when considering whether caged or cage-free eggs are better for the world, their environmental impact is not very high on the list of things to agonize over. 

Martinić and his co-authors don’t ignore animal welfare entirely — they attempt to account for it, but they have an oddly narrow way of doing it. The paper’s welfare metrics leave out what it is actually like for a hen to live inside a cage. Instead, its chosen measures score cage systems more favorably, because the model assumes that they have lower mortality rates, and they require fewer total birds to produce the same number of eggs. The result is that cage farming ends up looking comparatively better on welfare, though the authors themselves acknowledge that their analysis leaves out many indicators of welfare and “should be interpreted with caution.” (Critics of the study have noted that its mortality numbers are more than a decade out of date, and that recently, as egg producers have learned to better manage cage-free farms, the gap in mortality rates has declined.)

Beyond the carbon ledger

There is a term for what happens when we reduce our biggest social problems to environmental or climate ones, and lose sight of everything else at stake: climate reductionism. 

The new paper and discourse surrounding it display how that trap can shape the way we understand and treat nonhuman animals. It also reflects a long-established tendency in animal agriculture research to view animal welfare through the lens of efficiency and productivity, rather than through the desires of the animals themselves. 

To contend more deeply with the ethics of mass-producing animals like egg-laying hens for food, we have to momentarily cast aside questions about efficiency and ask, what is life like for a hen from her own perspective? I happen to know a lot of chickens — in the right circumstances, they’re convivial, spirited, energetic animals. The widespread myth that they’re stupid has more to do with our own repressed guilt than with the animals’ capacities. Their experiences call on us to face not just the cruelty of cages, but also to look beyond cages to problems common to all forms of egg farming. 

To name just a few: Egg-laying hens have been bred to channel their bodily resources into laying extraordinary numbers of eggs, which takes a heavy toll on their bodies, depleting their skeletons of calcium, and making them prone to painful bone fractures and reproductive disorders. (The red junglefowl, the wild animal equivalent of the chicken, lays 10 to 15 eggs per year; farmed hens lay more than 300.) On cage-free farms, meanwhile, escaping the cage does not mean entering anything resembling a natural environment. Rather, the hens live in crowded, polluted, sometimes windowless sheds filled with thousands or tens of thousands of other chickens, where unnaturally large, dense social groups can intensify injurious pecking and other aggression. They are ultimately killed once their productivity wanes.

These conditions have even fueled a vigorous debate in the animal advocacy movement over whether pushing for cage-free eggs is a good idea at all, because they can give the public the false impression that the worst parts of egg factory farming have gone away. But cages are only one part of that system — modern animal agriculture is defined by a complex set of practices optimized to squeeze as much productivity out of animals as possible. Challenging more fundamental features of egg production, like the genetics of modern laying hens, would be harder than eliminating cages because it poses a deeper challenge to the value the industry elevates above all else: efficiency. We would have to demand less of their bodies, and that would very likely make each egg more expensive and more emissions-intensive. 

What, then, does that mean for you? On an aggregate, societal level, I think cage-free reforms have been a good thing, because they have made a terrible system that we can’t simply wave away with a magic wand less bad. But more profound change will require consumers to demand something more than cheap animal products. So if I’m addressing you, an individual, I’d say something different. The most obvious win-win is hiding in plain sight: Plant-based foods like tofu and beans are significantly lower in emissions than eggs, while avoiding the moral costs of egg production altogether.