Climate change arrives in clinic as ordinary diagnoses at unusual times

The instinct, when you first hear that climate change is a health problem, is to look for a new disease. Something distinctive that a clinician could learn to recognise and add to a differential.

That is not how it presents. The elective's most useful correction, early on, was that climate change almost never produces a novel diagnosis. It produces familiar diagnoses, in unfamiliar people, at unfamiliar times of year, in unfamiliar places. Heat stroke in a temperate city. Dengue acquired without leaving Italy. An asthma exacerbation in a month that used to be quiet. The pathology is ordinary. The epidemiology is not.

That makes it a hard signal to see from inside a single consultation, because nothing about the individual patient looks unusual. You only see it in aggregate, or if you happen to be asking the right question.

Heat, which does the most and gets noticed least

Heat is the most direct pathway and the most consistently underestimated. Part of the reason is attribution: heat rarely appears on a death certificate. A person with heart failure who dies during a heatwave dies of heart failure, and the heat that precipitated it leaves no mark in the record.

Physiologically the burden falls where you would expect. Thermoregulation depends on cutaneous vasodilation and sweating, both of which cost cardiac output and volume. In someone with limited cardiac reserve, or on diuretics, or with reduced ability to concentrate urine, that demand is not free. Add anticholinergics that blunt sweating, or beta-blockers that limit the chronotropic response, and the margin narrows further.

The people most at risk are therefore the ones already on the most medication, often living alone, sometimes with the cognitive impairment that stops them recognising thirst. Southern Europe's 2003 heatwave made that pattern visible in a way that reshaped public health planning across the continent, and Italy remains one of the more exposed countries in the region.

Occupational exposure is the other half of it — outdoor and unairconditioned indoor work — and there the emerging concern is chronic rather than acute: repeated dehydration and heat stress appear to contribute to chronic kidney disease of non-traditional cause in agricultural workers in several hot regions. The mechanism is still argued over, which is worth saying plainly rather than glossing.

Vectors moving north

Aedes albopictus, the tiger mosquito, established itself in Italy in the 1990s and is now widespread. What has changed is not its presence but the length and warmth of the season in which it can transmit — warmer temperatures shorten the extrinsic incubation period, so a mosquito that bites an infected traveller is more likely to survive long enough to become infectious.

The consequence is autochthonous transmission of arboviruses in a country that has no tropical climate. Italy has recorded locally acquired chikungunya outbreaks, and locally acquired dengue cases have occurred in recent years. West Nile virus, carried by Culex, has circulated in the Po Valley since the late 2000s and now behaves as a seasonal expectation rather than an event.

The clinical implication is small but real: travel history alone is no longer sufficient to exclude these infections during the summer months in parts of Europe, and febrile illness in late summer deserves a slightly different differential than it did twenty years ago.

Air, allergens and the growing season

Two effects run in parallel. Warmer temperatures and stronger sunlight accelerate the photochemistry that produces ground-level ozone, which is directly irritant to airways. Separately, a longer and warmer growing season lengthens pollen seasons and, for some species, increases pollen production — so allergic rhinitis and allergic asthma get a wider window.

Wildfire smoke is the newer contributor, and its particulate load can travel very long distances from the fire itself. The health effects of fine particulate matter are among the best-established in environmental medicine, which makes this the least speculative part of the picture.

Water, food and displacement

Flooding contaminates water supplies and drives diarrhoeal disease; drought reduces crop yields and drives undernutrition, which in children is not only a nutritional problem but a developmental and immunological one. Both drive displacement, and displaced populations lose continuity of care, medication supply and vaccination coverage.

The mental health dimension is harder to quantify and easy to dismiss, which is probably why the elective spent time on it. Loss of home and livelihood produces the psychiatric burden that loss of home and livelihood has always produced. The newer phenomenon — sustained anxiety about the future among young people — has a thinner evidence base and I would not want to overstate it.

The part that implicates us

The section I found most uncomfortable, and therefore most worth recording, is that health systems are a significant emitter in their own right. Estimates put healthcare at roughly four to five per cent of global emissions — larger than aviation.

Some of that is unavoidable: hospitals are energy-intensive buildings running continuously. But a meaningful share sits in clinical decisions that are genuinely discretionary. Volatile anaesthetic agents differ enormously in their warming potential, and desflurane's is far higher than sevoflurane's for broadly comparable clinical use. Metered-dose inhalers use hydrofluorocarbon propellants; dry powder inhalers do not, and for many patients either would work. Unnecessary investigations carry a carbon cost alongside their more familiar costs in anxiety and false positives.

None of this is a reason to withhold indicated treatment from a patient in front of you. But where two options are clinically equivalent, one of them is not environmentally equivalent, and that is a straightforward thing for a prescriber to know.

What I am taking from it

The claim I would defend from this elective is narrow: climate change is a clinical subject, not only a policy one, because it changes the prior probabilities a clinician works with.

It changes who gets ill, when, and with what. It does not require a new specialty or a new vocabulary. It requires the exposure history to include heat and air quality, the travel history to include the local season, and the prescribing decision to occasionally account for something outside the room.

I am aware that some of the numbers above are ones I have taken on trust from lecture material and have not yet traced to source. The healthcare emissions figure in particular gets quoted widely and I would like to find the underlying methodology before I repeat it again. Noting that here so I remember to.