Sunlight is essential to life, but prolonged exposure can place significant stress on the human body.

When we think about summer heat, we often focus on air temperature. But standing in direct sunlight introduces another major environmental factor: solar radiation.

The sun can affect the body in several different ways.

Ultraviolet radiation can damage skin.

Visible and infrared radiation can contribute to heating.

Extended exposure can increase the body's thermal load, particularly when combined with high air temperature, humidity, physical activity and inadequate cooling.

This makes sun exposure more than a question of whether a day “feels hot.”

It is an interaction between solar radiation, skin, clothing, thermoregulation and the surrounding environment.

What Happens When the Body Is Exposed to the Sun?

When you stand outdoors under direct sunlight, energy from the sun reaches your body.

Some of this radiation is reflected.

Some is absorbed by the skin and clothing.

Absorbed radiation increases the thermal energy of the exposed surface.

The body then has to manage this additional heat alongside the heat it is already producing internally.

This creates a simple relationship:

Metabolic heat + environmental heat gain = total thermal load

The greater the thermal load, the more effectively the body needs to dissipate heat to maintain a stable internal temperature.

Sunlight Is More Than Visible Light

Solar radiation contains different wavelengths.

The main categories relevant to human exposure are:

• Ultraviolet (UV) radiation
• Visible light
• Infrared radiation

Each interacts with the body differently.

Ultraviolet Radiation

UV radiation has enough energy to cause biological changes in skin tissue.

UVA and UVB are particularly relevant to human health.

Visible Light

Visible wavelengths allow us to see and also contribute to solar energy reaching surfaces.

Infrared Radiation

Infrared radiation contributes to radiant heat exchange and can increase the thermal load experienced by exposed surfaces.

These components do not produce identical effects.

Understanding sun exposure therefore requires separating UV exposure from heat exposure.

UV Radiation and Sun Exposure

Ultraviolet radiation is invisible to the human eye.

This creates an important problem.

You can feel heat.

You cannot directly feel UV radiation.

A person may therefore associate intense sunlight primarily with temperature while underestimating UV exposure.

UV radiation can affect the skin even when the surrounding air does not feel exceptionally hot.

The amount of UV reaching the surface depends on several factors, including:

• Solar elevation
• Time of day
• Season
• Latitude
• Cloud conditions
• Atmospheric conditions
• Altitude
• Ground reflection

This is why temperature alone is not a reliable indicator of UV exposure.

UVA and UVB

The two types of UV radiation most relevant to everyday outdoor exposure are UVA and UVB.

UVA

UVA penetrates more deeply into the skin and contributes to photoaging and other long-term skin damage.

UVB

UVB is strongly associated with sunburn and also contributes to skin damage and skin cancer risk.

Both matter.

Sun protection should therefore be approached as a combination of reducing exposure and using effective protective measures.

The UV Index

The UV Index is a standardized measure used to communicate the strength of ultraviolet radiation at the Earth's surface.

Unlike temperature, the UV Index is specifically related to UV exposure rather than how hot the air feels.

A hot day does not automatically mean the UV Index is extremely high.

Conversely, significant UV exposure can occur even when temperatures feel relatively comfortable.

Checking the UV Index can therefore provide information that a normal weather forecast temperature cannot.

Why Direct Sunlight Feels Hotter

Have you ever noticed how quickly you warm up after stepping from shade into direct sunlight?

The air temperature may be almost identical.

The difference is radiant heat.

In direct sunlight, your body receives additional energy from solar radiation.

In shade, much of that direct solar input is blocked.

This is why shade can feel dramatically cooler even when the thermometer shows the same air temperature.

The distinction is important:

Air temperature describes the surrounding air.

Radiant exposure describes energy transferred through electromagnetic radiation.

Both influence the human thermal environment.

Shade Changes the Thermal Environment

Shade is one of the simplest ways to reduce direct solar exposure.

A tree, building, awning or other structure can block a substantial portion of direct solar radiation.

This reduces the amount of radiant energy reaching the body.

But shade does not necessarily eliminate heat.

The surrounding air can still be hot.

Nearby surfaces can still radiate heat.

Humidity can still restrict evaporation.

The body may still need to sweat heavily.

Shade should therefore be understood as one important component of heat management rather than a complete solution.

Clothing as a Physical Barrier

Clothing creates a physical layer between the body and the environment.

In direct sunlight, this layer can reduce the amount of solar radiation reaching the skin.

The effectiveness of clothing as a protective barrier depends on several properties.

These can include:

• Fabric construction
• Fabric density
• Thickness
• Porosity
• Colour
• Fibre characteristics
• Moisture content
• Coverage
• Garment geometry
• Stretch

A textile's appearance alone cannot tell you exactly how much UV it blocks.

This is why standardized measures such as UPF (Ultraviolet Protection Factor) are useful when UV protection is a specific requirement.

What Is UPF?

UPF stands for Ultraviolet Protection Factor.

It describes how effectively a fabric reduces UV radiation reaching the skin through the material.

For example, a garment with a UPF rating indicates that only a fraction of UV radiation passes through the fabric compared with unprotected skin.

UPF is different from SPF.

SPF is primarily used for sunscreen.

UPF is used for textiles.

A garment's UV-protective performance depends on its construction and can change with factors such as stretching or wetness, depending on the textile.

Does Dark Clothing Protect Better?

Colour can influence solar radiation absorption.

Darker materials generally absorb more visible and near-infrared radiation than lighter materials, potentially causing the garment itself to become warmer under strong sunlight.

However, UV protection is not determined by colour alone.

Fabric construction, density, fibre properties, dyes and finishing treatments can all affect UV transmission.

Therefore:

Dark does not automatically mean better UV protection.

And:

Light does not automatically mean better UV protection.

For reliable UV protection, the textile's actual performance is more important than colour alone.

Coverage and Breathability Are Different Problems

Summer clothing faces an interesting design challenge.

More coverage can reduce direct solar exposure to the skin.

But additional fabric can also influence airflow and heat retention.

A garment that provides substantial coverage but severely restricts ventilation may become uncomfortable in hot conditions.

Conversely, a highly open garment may provide excellent airflow while allowing more direct solar and UV exposure.

This creates a design balance:

Protection + airflow + moisture management + thermal comfort

The optimal balance depends on the environment and intended use.

The Importance of Fabric Structure

The structure of a textile determines how light, air and moisture interact with the garment.

Two fabrics made from similar fibres can perform differently because their:

• Weave
• Knit
• Density
• Thickness
• Porosity
• Yarn construction

may differ.

For UV protection, tighter and more opaque structures can generally reduce transmission compared with highly open structures.

For airflow, greater openness can increase air permeability.

These properties can therefore compete with one another.

This is one reason designing summer clothing is fundamentally an engineering problem.

The Solar Heat vs UV Protection Trade-Off

A garment can be designed to block UV without necessarily being optimized for thermal comfort.

Similarly, a highly breathable garment may allow more solar radiation through its structure.

These are separate performance objectives.

A useful summer garment therefore needs to consider at least two different questions:

Question 1

How effectively does the garment protect the skin from UV radiation?

Question 2

How effectively does the garment manage heat and moisture around the body?

The best solution depends on the intended climate and use case.

Sun Exposure and Sweat

Sweating is one of the body's primary cooling mechanisms.

But direct sunlight can increase the thermal load on the body, potentially increasing the demand for cooling.

If sweat evaporates efficiently, evaporation can remove heat.

If humidity is high, evaporation becomes more difficult.

This creates an important interaction:

Sunlight increases heat gain → body increases cooling response → sweating increases → evaporation becomes important.

The surrounding humidity and airflow then influence how effective that cooling process can be.

Humidity Makes Sun Exposure More Difficult

Imagine two people standing under the same sunlight.

One is in a dry environment.

The other is in a hot and humid environment.

Both receive similar solar radiation.

But their bodies may have different abilities to lose heat.

In the humid environment, sweat evaporation is more restricted.

The body may therefore experience greater difficulty dissipating the combined heat load.

This is why solar radiation, temperature and humidity need to be considered together.

Airflow and Sun Exposure

Air movement can change the way the body interacts with hot outdoor conditions.

In suitable conditions, airflow can:

• Remove warm air from around the skin
• Support sweat evaporation
• Reduce moisture accumulation
• Improve ventilation within clothing

But airflow is not always cooling.

If the surrounding air is hotter than the skin, strong hot air movement can increase heat transfer toward the body.

Therefore, airflow must be understood in relation to air temperature, humidity and skin temperature.

Why Urban Sun Exposure Is Different

Urban environments create multiple sources of radiant and thermal exposure.

Concrete, asphalt, glass, vehicles and building surfaces can absorb solar energy and later release it as heat.

A person walking through a city may therefore encounter:

• Direct sunlight
• Reflected sunlight
• Hot pavement
• Radiant heat from surrounding surfaces
• Vehicle heat
• Limited shade
• Reduced airflow

The result is often a more complex thermal environment than the simple air-temperature reading suggests.

This is particularly relevant during long commutes or outdoor work.

Reflected Sunlight Matters Too

Solar radiation does not only arrive directly from the sun.

Surfaces can reflect portions of incoming radiation.

The amount of reflection depends on the surface material and wavelength.

Light-coloured surfaces, water, sand and certain building materials can contribute to reflected radiation.

This means that standing near a reflective surface can change exposure even when the sun is not directly overhead.

Sun Exposure During Travel

Travel can increase cumulative outdoor exposure.

Walking between transport connections, waiting for vehicles, sightseeing, outdoor dining and exploring unfamiliar cities can all increase time spent under the sun.

Travel clothing therefore needs to balance:

• Comfort
• UV protection
• Coverage
• Breathability
• Packability
• Movement
• Style

The ideal garment depends on the destination and climate.

A jacket designed for a hot, dry environment may require different characteristics from one intended for a hot, humid city.

Sun Exposure and Physical Activity

Exercise increases metabolic heat production.

When exercise takes place outdoors under direct sunlight, the body has to manage both:

Internal heat production + environmental heat gain

Sweating and skin blood flow increase as the body attempts to maintain thermal balance.

The combination of intense activity, high temperature and direct sunlight can therefore increase heat stress.

For outdoor exercise, appropriate scheduling, shade, hydration, rest and environmental awareness are important.

Why Morning and Afternoon Exposure Can Differ

Solar radiation changes throughout the day.

The sun's position in the sky affects the amount of UV reaching the Earth's surface.

UV exposure is often strongest around the middle portion of the day when the sun is higher, although the exact pattern varies by location and season.

This means that outdoor planning should consider more than temperature.

A cooler morning can have a very different UV environment from a hotter afternoon, and local UV Index information provides a more useful indicator of UV intensity.

Sun Protection Is a Layered Strategy

No single method provides complete protection in every situation.

Effective sun protection can combine:

Shade

Reduces direct solar exposure.

Clothing

Creates a physical barrier over the skin.

Sunscreen

Provides topical protection for exposed skin when used appropriately.

Timing

Reducing exposure during periods of stronger UV can lower cumulative exposure.

Sunglasses and Head Protection

Can help protect sensitive areas from sunlight.

The appropriate strategy depends on the environment and duration of exposure.

Clothing Should Not Be Treated as a Complete Sun-Safety System

A garment can provide useful coverage.

But its performance depends on its construction, and exposed areas of the body remain exposed.

A breathable summer jacket also does not eliminate the risks associated with prolonged heat.

For this reason, clothing should be viewed as one component of a broader sun- and heat-management strategy.

This distinction becomes especially important in extreme summer conditions.

Designing the Modern Summer Jacket

A summer jacket intended for intense sunlight has to solve several competing problems.

It should ideally consider:

Solar exposure

How much direct radiation reaches the wearer?

UV transmission

How much UV passes through the textile?

Air permeability

How easily can air move through the garment?

Moisture transport

Can sweat and water vapour move away from the skin?

Garment geometry

Does the silhouette encourage ventilation?

Thermal load

Does the garment trap or release heat?

These parameters cannot always be maximized simultaneously.

The design process is therefore about finding an appropriate performance balance.

The Future of Sun-Responsive Clothing

As climate conditions become more challenging, clothing can increasingly be viewed as environmental equipment as well as fashion.

Future garments may incorporate technologies and construction methods designed around:

• UV protection
• Solar reflectance
• Air permeability
• Moisture transport
• Thermal regulation
• Adaptive ventilation
• Lightweight construction

The broader shift is from seasonal fashion to climate-responsive design.

Instead of asking only what looks appropriate for summer, designers can ask:

What does the human body need from clothing in this environment?

The Airlayer Perspective

At Airlayer, we think about summer clothing as a thermal interface between the human body and its environment.

Sun exposure makes that relationship particularly clear.

A garment sits between the body and solar radiation while simultaneously interacting with air, sweat, humidity and temperature.

The challenge is not simply to add more fabric.

It is to understand how coverage, UV protection, airflow, moisture management and thermal comfort interact.

A summer jacket should therefore be designed around the actual conditions in which it will be worn.

Because the sun does not simply illuminate the environment.

It changes the thermal environment itself.

Final Takeaway

Sun exposure is more than a question of getting a tan or feeling warm.

Solar radiation can contribute to both UV exposure and thermal load, affecting the skin and increasing the amount of heat the body must manage.

The impact depends on:

UV radiation + solar intensity + air temperature + humidity + airflow + clothing + duration of exposure

This is why effective summer protection requires a layered approach.

Use shade when possible. Protect exposed skin appropriately. Consider the UV Index. Stay aware of heat and hydration. Choose clothing suited to the climate and activity.

And when designing summer apparel, look beyond the label.

The important question is not simply:

“Is this a summer jacket?”

It is:

“How does this garment interact with sunlight, air, moisture and the human body?”

That question sits at the intersection of sun protection, textile science, thermoregulation and climate-responsive fashion.