Climate: whatever tends to happen outside, against which we humans can barricade ourselves by building structures that are safe, controlled environments. Right? Not completely... (Sorry for dashing the dreams of the would-be Mars colonists here.) While this definition certainly encompasses most of what is typically meant by the word 'climate', indoor spaces don't exist in a vacuum -- they're more controlled and cordoned off than the free-flowing outdoors, but they can still vary substantially, between regions as well as between individual buildings. These variations in indoor environments are amplified by the large amounts of time that we spend there -- about 87% of our lives, plus another 6% in vehicles. The fact that what we term 'the indoors' is essentially an archipelago of millions of isolated structures, each with its own distinct combination of materials, usage, occupants, contents, and ventilation, in some ways makes indoor climate study as challenging as its outdoor counterpart.
First of all, indoor climates demand a different set of tools. While outdoor climate is influenced by familiar factors such as solar radiation, winds, clouds, vegetation, and greenhouse gases, the workings that give rise to indoor climates (air ventilation and materials composition, for instance) are often deliberately hidden from view. Aspects of them are controllable to varying degrees: temperature in a house may be mostly steady during winter, for example, but plunge when power is lost. Just like its analogue in the 'outside' world, this short-lived extreme event can have outsize consequences, from damage to infrastructure to health effects on vulnerable people. Indoor environments are also strongly shaped by the interstitial spaces between outdoor and indoor, whether large enough to accommodate people (i.e. doors) or too tight for the smallest insect.
To a large degree, of course, indoor and outdoor climates correlate across space and time. But since the indoors is where most people spent the great majority of their time, whether and how the outdoors comes in deserves considerable interest. This can be modulated by physical aspects of a building, the behavior of its inhabitants, and its immediate outdoor microclimate as noted in a paper from several years ago. For instance: is there A/C and/or reliable heating? Is the building shaded? Are there stoves or filters? How good is the ventilation? Do any of the inhabitants smoke?
An extensive indoor/outdoor air pollution study in rural China, contributed to by my friend Ting Zhang, found generally similar or somewhat more elevated levels of pollutants indoors vis-à-vis outdoors. The primary difference was in certain secondary pollutants, which were up to 50% more numerous indoors due to reactions of normal trace air constituents (like ozone) with household chemicals. With its finding of a substantial influence of coal and wood combustion on indoor pollution levels, this study was representative of large parts of the developing world. Given the naturally constricted spaces of buildings, and frequent poor ventilation, smoke from indoor sources is associated with many respiratory and cardiovascular illnesses -- and for tobacco smoke, even "semi-open" spaces trap air enough to be detrimental to health. Particulates emitted from building materials and other indoor sources can also be problematic by accumulating in ways they don't outdoors (even if the emission occurs naturally), as indoors the ratio of pollutant to total air volume can be much, much greater. The obverse is that outdoor sources of pollution have to make it past the barrier of walls and doors, and to fight filtration or other systems. But when those hurdles are minimal, indoor pollution can reach worrisome levels.
In an attempt to quantify indoor heat exposure, a recent paper noted that during heat waves indoor dewpoints increase about 0.66 C for every 1.0 C increase in outdoor dewpoint, while for temperature the corresponding figure is only 0.20 C. In a survey of buildings in New York City a wide range of indoor heat indices were found, with a substantial percentage exceeding various heat-advisory levels (see figure above). While the numbers aren't anything outrageous, not taking sufficient precautions or otherwise underestimating these risks because they occur in people's homes or offices rather than out in public -- where environmental factors are more natural to consider -- could result in problems for sensitive groups. In particular, the common advice during periods of hazardous conditions (whether a snowstorm, a hurricane, extreme heat, extreme cold, etc) to 'stay home and stay safe' may need to be qualified to ensure that safety is actually achieved.
Indeed, I would argue that there's no such thing as complete liberation from sensitivity to climate -- it's just a matter of which kind of climate a person chooses to be exposed to at a given time. And we have to keep in mind that our choices have consequences that may not be evident at first blush, on ourselves or on the outdoor climate. But on a more-positive note, indoor climates are arguably more easily controllable (physically and regulatorily), and in the past half-century or so this has given rise to substantial decreases in heat-related mortality, as well as immeasurably improved levels of comfort for those able to afford all of the 21st century's most-advanced luxuries. Whether these improvements in indoor climate (and their attendant economic requirements) can extend to all people, while maintaining some semblance of a balanced and near-natural outdoor climate, is in my mind one of the century's big questions.