To real estate developers, "location, location, location" is a favorite motto. Architects like to talk about "commodity, firmness and delight." But as a building occupant, your greatest interest might be "health, safety and welfare."

All would agree that the interior environments of buildings should be reasonably hygienic and comfortable. This is not always easy to achieve. Since the 1800s, industrialization and its byproducts -- from atmospheric pollutants to radioactive contaminants in water and soil -- have compromised the quality of the earth, air and water, which were believed by the ancients to be the fundamental elements (along with fire).

Today, you may wonder whether you're better off indoors than outdoors. Should you open or close your windows? Should your house be sealed tightly or be slightly leaky?

Inside buildings, you could be assaulted by invisible, carcinogenic fibers that otherwise keep you warm. Radon could be seeping up from the ground, or all the synthetics around you could be emitting complex gaseous compounds molecule by molecule. Cigarette smoke, exotic molds and mildew and radiation from microwave ovens or computer video screens may be zapping you.

Outdoors, automobile exhaust fumes, smog, pollen, dust, countless bacteria and more fibers await. Even the sun's ultraviolet radiation can be harmful.

Back inside, your concerns include not only the cleanliness of the air, but also its temperature, humidity and movement. Few climates provide natural, year-round comfort conditions that make buildings habitable without heating, cooling or ventilation.

Until the 19th century, environmental-control technologies were very limited, but straightforward. In winter, space heating was provided directly by fireplaces or stoves. Heat was conserved by wearing ample clothing, sleeping under piles of blankets, and closing doors and windows in thick, uninsulated masonry or wood walls. And dampness was always a problem.

In summer, clothes were loosened while doors and windows were opened to emit breezes and encourage cross-ventilation. Roofs, walls and shutters kept out the hot sun, but not the oppressive humidity.

The industrial revolution brought advancements in engineering, especially metallurgy and thermodynamics, leading to "mechanical" heating systems. Boilers could burn wood, coal or oil to produce steam that could be distributed through pipes to room radiators.

As steam cools and condenses, its heat transfers to metal radiator sections. They, in turn, transfer heat in two ways: by infrared radiation from the metal radiator surfaces to other surfaces in the room, and by direct conduction of heat to air touching the radiator. Air warmed by conduction then "convects" upwards as cooler air falls downwards (a "convection loop").

Steam-heating systems had a limited impact on the exterior appearance of buildings. But inside, they necessitated a sooty boiler room in the basement, a network of clattering pipes attached to the building structure, and radiators situated under or adjacent to windows, which invariably were obstacles to laying carpets, placing furniture and hanging drapes.

Radiators also were noisy, leaky and rusty, and often made rooms too warm and dry, or not warm enough. The only control consisted of a valve regulating the quantity of steam entering each radiator, obliging occupants to act as human thermostats. Periodically, boilers would break down, valves would stick or pipes would burst, usually during severe cold spells.

The turn of the century witnessed the advent of commercially produced electricity, electric controls and motors, and refrigeration. Engineers realized that they could maintain complete control over the interior environment of a building. Indeed, with electric lighting, windows suddenly seemed optional.

Designers dreamed of creating idealized, sanitized, automated environments requiring little or no human intervention. Buildings would be enclosed hermetically with curtain walls and windows that didn't have to open, keeping out unwanted "ethers." In fact, human intervention seemed undesirable because it could upset the delicate balance and performance of centralized, mechanized, finely tuned systems.

Thus, several generations of hermetic buildings were erected, a few before World War II, but most since the war. Architects, engineers, building owners and occupants shared a common faith in the reliability of motors, fans, furnaces, ducts, compressors, filters, humidifiers and thermostats.

Until the 1970s, this design credo was reinforced by its economic feasibility. Energy was cheap and plentiful. Conserving energy through judicious building design was of low priority, even though mechanical systems -- the "hardware" -- represent a substantial portion of a building's construction budget, sometimes more than any other single system.

Fuel (or electricity) costs for heating and cooling are proportional to the amount of heat supplied to a building in winter and removed in summer. This in turn depends on:

*The comfort level desired by owners and occupants.

*The building's "skin" area, determined by its basic geometry.

*The amount of insulation in the skin (walls and roof).

*The quantity of exterior glazing and its insulating value.

*The building's tightness and resistance to air infiltration.

*The efficiency of its mechanical system in transferring heat.

But for many decades, fuel costs were not serious economic constraints on design. Architects and their clients willingly and generously glazed buildings, primarily for expressive, aesthetic reasons. Building compactness, double glazing, extra insulation and solar shading were not considered necessities.

Orientation also affects heat gain and loss. North sides of buildings are coldest in winter and coolest in summer, when south and west sides are hottest. Yet these differences often were disregarded in designing facades. Standardization, repetition of modular wall components and belief in the potential of the universal "skin" became part of the modern design ethic. Such an ethic, appropriate or not, depended in part on the existence of energy-intensive, mechanized environmental-control systems.

In Washington, New York, Los Angeles, Chicago, Houston, Miami or any other American city, you see countless modern buildings with windows that don't open. There are thousands of buildings with identical facades, regardless of orientation, buildings that consume large amounts of energy to heat and cool. And when fuel costs rose dramatically in the 1970s, operating costs and rents climbed correspondingly.

Over the past dozen years, designers have become more sensitive about methods and costs of comfort control. Buildings now are better insulated, with insulating glazing almost standard. Glass is used more sparingly. Orientation and solar conditions frequently influence the composition of facades.

Central heating and cooling systems have been decentralized, allowing occupants to control air temperature at many different points within a building. Thermostats can be programmed to maintain acceptable comfort conditions only during selected periods of building use.

Some mechanical systems provide simultaneous heating and cooling. Heat can be removed from one part of a building in need of cooling and transferred to another part of the building where heating is required. Recapture of heat generated by electric lights, and capture of heat from the sun, can reduce energy demand significantly. A building's structure, plus water or other "mass" storage, can act as heat sinks in winter.

Still, conserving energy, cutting fuel costs and creating climatically responsive architecture do not complete the environmental picture. Questions about atmospheric pollutants and biochemistry and fibrous carcinogens and background radiation remain. If the power fails, it's nice to be able to open a window to maintain comfort. But is it always healthy?

NEXT: The architecture of comfort systems.