Welcome to chapter twelve. Almost every number in this book so far has quietly described one person: a healthy adult of working age, near the middle of the distribution. Most of the people who use healthcare are not that person. Hospitals are full of the very young and the very old, and of people whose thinking, attention or mood is affected by illness, medication or stress. This chapter widens the range you design for.
A design that works for the person who built it may fail a three-year-old, an eighty-five-year-old, or anyone on the worst day of their life, and it will usually fail them in ways you did not think to test. This chapter covers three groups. Children, whose proportions, physiology and behaviour differ in ways that change what is safe. Older adults, whose bodies change system by system, gradually and very unevenly. And the mind: attention, memory, cognitive load, dementia and delirium, mental health, neurodiversity and stress. The thread is a principle from chapter seven: design for the range of real users, not the middle of it.
We start with children.
A newborn weighs about three and a half kilograms and is about fifty centimetres long, and its weight triples by the first birthday. But growth is not uniform enlargement. The head is about a quarter of body length at birth and an eighth in an adult, so a baby's centre of mass is high, and the large back of the skull flexes the neck when the baby lies on its back. Limbs are short and lengthen through childhood. And a newborn has two to three times as much skin surface per kilogram as an adult, so heat and water move proportionally faster. A device scaled to seventy percent of adult size fits no actual child, because no child is a seventy percent adult in every dimension.
The infant airway differs in ways that make it easier to obstruct: a large tongue, a higher larynx, and young infants breathe mainly through the nose, so a blocked nose matters far more. Above all, the airway is narrow, and resistance to flow rises with the inverse fourth power of radius. Take an infant airway about four millimetres across. One millimetre of swelling all round halves the radius, and two to the fourth is sixteen: sixteen times the resistance. The same millimetre in an adult airway sixteen millimetres across raises resistance about one point seven times. Anything near a child's face, nose or neck deserves that arithmetic.
Heart and breathing rates fall with age. A newborn's heart typically runs at a hundred and ten to a hundred and sixty beats a minute and breathes thirty to sixty times; an adult at sixty to a hundred and twelve to twenty. These are orientation ranges, and published paediatric ranges differ, which is exactly why clinical tools state their reference. The design point is that an alarm limit is a function of age. A heart rate of a hundred and fifty is unremarkable in a newborn and alarming in an adult. Software that displays, flags or simulates vital signs without an age-appropriate reference is wrong for every child it meets.
Other differences matter as much. Body water is seventy-five to eighty percent of a newborn's mass, and fluid turns over quickly, so dehydration develops fast. Blood volume is around eighty to ninety millilitres per kilogram, so a three and a half kilogram baby has only about three hundred millilitres of blood; repeated samples are a real fraction of it. Newborns cannot shiver effectively. They make heat from brown fat and lose it across their large surface, which is why incubators exist. And newborn skin, especially in babies born early, is thinner and less of a barrier. Removing a sensor's tape can strip the outer layer, a recognised design concern for every sensor that sticks on.
Children change in what they do. A fine pincer grasp arrives around nine or ten months, mouthing is most intense in the first two years, and soon they climb, open and unscrew everything. Capability races ahead of judgement: a small part a six-month-old cannot pick up, a two-year-old can swallow. Toy and nursery rules on small parts, cords and finger traps all come from that race. A child cannot usually consent, so a carer does, but good practice also seeks the child's own assent, and any contact with children goes through safeguarding. Paediatric devices are a field of their own: everything is calculated per kilogram, so a decimal slip gives a tenfold error; many sizes must be stocked; and small markets mean much is adapted.
Pregnancy changes almost every system. Blood volume rises by roughly forty to fifty percent and cardiac output by a similar amount. Lying flat on the back in later pregnancy can let the uterus compress the large vessels returning blood to the heart, so pregnant people are positioned tilted to one side, and a couch or positioning aid that assumes a supine adult is not neutral. Ligaments soften and the centre of mass moves forward, so falls are more likely. Breathing changes, and imaging with ionising radiation is weighed especially carefully. Two people are affected by any intervention, and most devices and drugs are validated on neither.
Now older adults.
Ageing is not a disease, and two eighty-year-olds can differ more than two thirty-year-olds. What ageing reliably reduces is reserve, the margin between everyday load and maximum. Skin thins and its layers separate easily, so adhesives and edges cause tears. Muscle mass and power fall, so operating forces must be low. Bone thins, so a minor fall can break a hip. Presbyopia is universal and high-frequency hearing fades, so text must be larger and alarms lower-pitched and visual. Gait slows, kidney clearance falls so drugs accumulate, and processing speed slows. Dementia becomes more common, but it is not normal ageing.
Frailty is reduced reserve across several systems, so a minor event causes a disproportionate decline. One widely used definition counts five features: weight loss, exhaustion, weakness, slow walking and low activity. Two of them, grip strength and walking speed, are measurements. Falls feed a vicious cycle: a fall, fear of falling, moving less, muscle and bone loss, weakness, and another fall. About a third of people over sixty-five living in the community fall each year. Many contributors are environmental: poor lighting, loose cables, slippery floors, furniture at the wrong height. That is the maker's opening, and a reminder that a charging cable trailing from a bedside device is a fall waiting to happen.
Older adults often live with several long-term conditions and take five or more medicines. Each may be reasonable alone; together they interact, and ageing changes how drugs are handled. Medication management is a daily task with real failure modes, and packaging that arthritic hands can open but children cannot is a genuine tension. A guideline written for one condition may not fit a person with five. For design: size text for the real viewing distance; use lower-pitched sounds, always duplicated visually; keep forces low with no fine pinch and no two-handed actions; avoid short timeouts; and respect the user. Older adults are adults, and designs that look childish are rejected, often rightly.
Now the mind.
Three parts of cognition matter most. Attention is limited, captured by anything sudden or loud, and sustained attention to a dull task declines within tens of minutes: the vigilance decrement, one reason monitoring by eye alone fails. Working memory, the mental scratchpad, holds around four meaningful chunks, and an interruption can wipe it. Executive function, planning, sequencing and inhibiting the wrong action, is the first to fail under fatigue, stress and illness. An interface that makes users carry a code between screens or convert units in their heads spends working memory a stressed user may not have. Put knowledge in the world, show the current state, one action per step, prevent errors, and allow recovery.
Dementia and delirium are often confused. Dementia is gradual and progressive, with attention often preserved early. Delirium is sudden, fluctuates, impairs attention, and is usually reversible if its cause, often infection, medication, dehydration or pain, is treated. Delirium is a medical emergency, common in older people in hospital and often the quiet kind, a person who becomes withdrawn and drowsy. Some of what helps is low-tech: visible clocks, good lighting and darkness at night, glasses and hearing aids actually in use, familiar objects, few ward moves. Dementia-friendly design overlaps with good design for everyone: clear contrast, no shiny floors that look like water, pictorial signs, and controls that look like what they are.
Mental health conditions are common, around one adult in five in a given year in many surveys, and they coexist with physical illness far more often than chance predicts. Language matters: say a person with schizophrenia, not a schizophrenic; died by suicide, not committed suicide. A learning disability is a reduced ability to understand complex information and learn new skills, starting in childhood, and people with learning disabilities experience documented health inequalities. Many autistic people have sensory differences. Design consequences are concrete: easy-read information with pictures, predictable sequences, the option to preview, control over sound and light. A noisy, bright clinical environment is itself a barrier.
Clinical staff and patients often operate under stress. Moderate arousal can sharpen simple tasks, but high stress narrows attention, shrinks working memory, and pushes people toward habitual actions: right if the habit fits, wrong if this device works differently from the one they trained on. The inverted-U picture of arousal and performance is useful rather than precise. For anything used in an emergency, make the critical action the most obvious one and the dangerous action hard to do by accident; keep conventions consistent with what users know; reduce alarms to those that need action; and assume interruptions, making it easy to see where a task was left.
In inpatient mental-health settings, a leading method of suicide is hanging, often from ordinary fixtures: door handles, hinges, rails, hooks, taps and window fittings. A ligature point is anything a cord could be looped around that could bear a person's weight; a ligature is anything that could be the cord. Mental-health buildings follow anti-ligature principles: rounded, sloped and flush surfaces, fittings that collapse or release under low load, tamper-resistant fixings. For you this is practical. Any object for a mental-health ward, a training model, a sensory aid, a device with a cable or strap, must go through the ward's ligature risk assessment before it is introduced.
Now the engineering bridge.
Many physiological quantities do not scale with body mass. Metabolic rate scales roughly with mass to the three-quarters power, and heat loss with surface area, so some doses are normalised to body surface area. The Mosteller formula estimates it as the square root of height in centimetres times weight in kilograms, divided by thirty-six hundred. An adult of a hundred and seventy centimetres and seventy kilograms has about one point eight square metres. A newborn of fifty centimetres and three and a half kilograms has about point two two. Per kilogram, the newborn has about two point four times as much surface, so it loses heat and water faster. Per kilogram is a modelling choice, and software must state which model it implements.
Growth charts are distributions. Being on a low centile is not by itself abnormal; by construction two percent of healthy children lie below the second. The trajectory matters more: a child falling from the seventy-fifth to the ninth centile needs attention. Design across the range: excluding five percent at each end of one dimension excludes ten percent, and across several dimensions far more, mostly children, older and disabled people. Hick's law says decision time grows with the logarithm of the number of options, but its slope grows with age, fatigue and stress. Any task that needs more than about four new items held in mind will fail for some users, and recognising beats recalling.
Falling converts potential energy into kinetic energy. A seventy kilogram person whose hip drops about nought point nine metres in a sideways fall carries about six hundred and twenty joules. The force depends on how far the impact is stopped over; on a hard floor that is a few centimetres of soft tissue, and peak force can reach several kilonewtons, enough to break an osteoporotic femur. The engineering responses work on the variables. Reduce the height: a fall from a twenty centimetre low bed carries well under a quarter of the energy of one from ninety. Increase stopping distance with hip protectors and impact-absorbing flooring. Prevent the fall. And shorten time on the floor with detection and alerting.
The devices range from incubators and length-based weight tapes to hip protectors, low beds, fall detectors, medication dispensers and anti-ligature fittings. The best entry points are informational and environmental: easy-read patient information, better labelling, ward clocks and signage, accessible controls, and replacement parts for paediatric manikins. They draw on the whole toolkit, writing and graphic design, signage and sheet materials, fabrics, electronics and software. Three boundaries are firm. Anything a child could reach must be assessed for small parts, cords, entrapment, button cells and magnets. Anything that bears weight for a frail adult is a prescribed device. And nothing enters a mental-health ward without its ligature risk assessment.
Two requests arrive in the same week for your elbow model: a play specialist on a children's ward, and a therapist running a falls-prevention group. For the children: any part that fits the small-parts cylinder must be captive or redesigned, and coin cells are out. The cord is a strangulation risk, so shorten it, make a head loop impossible, or use a breakaway. The hinge closes like scissors, so guard the finger traps. Make it wipeable, with picture labels. For the older group: size labels for one to two metres and six-twelve vision, a whole-hand pull tab with low force, one action with a visible result, and a look of equipment rather than a toy. Both came from one question asked before changing a part.
For the exercises: calculate body surface area for yourself, a newborn and a ten-year-old, per kilogram, and explain the curve. Plot airway resistance against radius and mark the infant and adult. Calculate fall energy from a bed, a chair and standing hip height, and list the trip hazards in one room. Audit a multi-step task for how much the user must hold in mind, and redesign it to two items or fewer. Rewrite a paragraph of instructions in easy-read style and test it on someone. And walk your own home looking for load-bearing fixtures something could be looped around, simply to see how ordinary they are.
The limits. You can follow the conversation and design more inclusively, but assessing a child's development, an older person's frailty, or anyone's cognition or mental health is clinical work. The scaling arithmetic shows why software must be careful, not how to dose. Anti-ligature design is a specialist field and every ward makes its own assessment. Simulation suits help find barriers but do not tell you what it is like to be old, disabled or unwell. And work with children or with people who may lack capacity has legal and safeguarding requirements your host establishes and you follow without exception. Next, infection control and choosing materials.