Indoor lighting

Insolation: hygienic significance and standards

Natural light is not only about visual comfort, but also a powerful factor in disease prevention. The key concept here is insolation. From a hygienic perspective, the sun performs vital health-improving work: it brings light energy, heat, and ultraviolet radiation. Ultraviolet rays possess a pronounced bactericidal effect, significantly reducing microbial contamination of the dwelling, and also have a beneficial effect on the nervous system and mood.

For the sun to bring maximum benefit, strict continuous insolation norms are established, which are calculated during design. They depend on the geographical latitude where the building is constructed:

• Northern zone (north of 58° N): the sun must shine into the windows for at least 2.5 hours per day (taking into account the period from April 22 to August 22).
• Central zone (from 48° to 58° N): at least 2 hours per day (from March 22 to September 22).
• Southern zone (south of 48° N): 1.5 hours is sufficient (from February 22 to October 22).

In a standard 1–3 room apartment, sunlight must reach at least one residential room. In larger apartments (4 or more rooms), there must be at least two insolated rooms. If the sun hits the windows intermittently, the total time norm is increased by 0.5 hours. In exceptional cases (e.g., dense historical development or reconstruction), reducing the norm is permitted, but by no more than 30 minutes.

Choosing window orientation considering the climate

The main rule for most residential buildings is to face the windows of main rooms to the south or southeast. This is how to capture the maximum amount of direct rays. However, climatic and geographical features introduce adjustments to architecture:

1. In a hot climate, the sun falls almost vertically. South-facing windows are ideal—the rays merely glide along the glass without causing critical overheating. However, a western orientation is strictly prohibited: the sun will shine into the windows in the second half of the day when the walls and air are already maximally heated, leading to unbearable heat. Shutters, blinds, and covered balconies are also used for protection.
2. In cold regions, the southern side is also a priority, but for a different reason. There, the sun is low, and inclined rays penetrate deep into the room, warming it. It is also important to hide facades from prevailing icy winds by turning blank end walls toward them.
3. In temperate latitudes, the meridional orientation (east-west) or placement along the heliodermal axis (deviation from the meridian by 19–22°) is considered optimal.

Interestingly, direct sunlight is not needed everywhere. If an operating room, dental office, or art gallery is being designed, the sun will only interfere with its blinding glare and harsh shadows. Windows in such specific rooms are always oriented to the north.

Interior finish: how color affects illumination

How bright a room appears is directly influenced by its interior finish. Dark or dirty walls can absorb the lion's share of natural lighting. Each color has its own reflection coefficient for incident light:

• White color is the absolute champion, returning up to 80% of light.
• Light yellow and light green reflect 60% and 46%, respectively.
• Dark green and dark blue absorb almost everything, reflecting only 10% and 6%.

It is important to remember that contaminated walls reflect light twice as poorly as freshly painted ones. Ceilings are traditionally painted white specifically for maximum reflection. A soot-covered ceiling immediately reduces the overall illumination of the room by about a third.

When choosing textures, hygienists strongly recommend using matte surfaces (e.g., glue-based paints). They softly scatter light, providing diffuse reflection. Glossy and lacquered surfaces create a mirror effect—harsh glare—which harms vision and tires the eyes.

From a psychophysiological standpoint, shades close to the solar spectrum and nature are considered ideal: red-orange and yellow tones invigorate and give the illusion of warmth, while green ones calm. It is the combination of green and yellow that least fatigues the visual analyzer.

Influence of room shape on light distribution

For light to evenly fill the space, the geometry of the room itself is important. The optimal shape of a living room should approach a square—this is convenient for visual perception, furniture arrangement, and proper illumination.

The maximum permissible room depth (the distance from the window to the opposite wall) is no more than 6 meters. Creating deep niches or alcoves far from the window is strongly discouraged: sunlight does not reach there, ventilation is disrupted, and zones with a poor microclimate form, unsuitable for healthy sleep.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Hygienic assessment of polymer building materials
• Thermal conductivity and hygroscopicity of building structures
• Influence of the number of storeys and layout on the microclimate of apartments
• Prevention of dampness and control of dry rot (Merulius lacrimans)

Other topics in the section "Chapter 4. Ecological and hygienic problems of populated areas and housing"

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.