When the sun is obscured by a thick layer of smoke, such as from wildfires, many people wonder if they can still acquire a tan. The visual dimming of the sun suggests a reduction in solar intensity, leading many to believe that the risk of sun exposure is eliminated. However, the physical interaction between smoke particles and ultraviolet radiation is complex and does not offer complete protection. This article explores the scientific mechanisms involved to determine the tanning potential and associated risks under a smoke-filled sky.
Understanding Ultraviolet Radiation and Tanning
Tanning is a biological defense mechanism triggered by exposure to ultraviolet (UV) radiation from the sun. The process involves specialized skin cells called melanocytes producing the pigment melanin, which absorbs UV rays and dissipates the energy as heat, protecting the underlying tissue. This darkening of the skin is a sign that cellular damage has occurred and the body is attempting to safeguard itself against further harm.
Solar UV radiation reaching the Earth’s surface consists mainly of UVA and UVB rays. UVB radiation primarily affects the epidermis, the skin’s outer layer, and is the main driver of sunburn and the delayed tan that appears days after exposure due to the synthesis of new melanin. UVA rays penetrate more deeply into the dermis, causing damage to collagen and elastin fibers, and they produce an immediate tan by oxidizing existing melanin in the skin. Both types of radiation contribute to the cumulative damage that can lead to premature aging and other long-term skin health issues.
Smoke Particles and UV Light Blocking
Smoke, particularly from large-scale events like wildfires, is an aerosol mixture composed of gases and fine particulate matter. These microscopic solid particles, including black carbon and organic aerosols, reduce the amount of sunlight that reaches the ground. The interaction between these particles and UV light occurs through two primary atmospheric processes: scattering and absorption.
Scattering involves the smoke particles redirecting the UV photons in various directions, preventing them from traveling directly to the surface. This mechanism is effective against the shorter wavelengths of UV light. Absorption occurs when components within the smoke, such as black carbon and brown carbon (a type of organic aerosol), physically take in the energy of the UV radiation.
The presence of UV-absorbing organic materials in wood smoke contributes to the reduction in the UV index (UVI) at ground level. In areas with very thick smoke plumes, the UVI can be reduced by more than 50% compared to clear-sky conditions. However, the effectiveness of this blocking depends on the smoke’s density, the composition of the burning material, and the altitude of the plume. While the overall UV exposure is lessened, a substantial amount of radiation can still penetrate the haze.
Practical Tanning Risk Under Smoke Cover
The assumption that heavy smoke eliminates the risk of tanning or sun damage is incorrect because the reduction in UV is partial, not absolute. Even when the sun appears as a dull, reddish disc, the atmosphere transmits enough UV radiation to still pose a risk. Studies measuring UV levels under smoke cover show that while the intensity is lower, the radiation does not disappear.
A key factor in the residual risk is the difference in how smoke particles interact with UVA and UVB rays. While both are reduced, the longer-wavelength UVA radiation, which contributes to tanning and aging, is less effectively blocked than the shorter-wavelength UVB. This means that a significant portion of the radiation responsible for skin damage and the tanning response is still getting through.
Factors like the time of day and the physical thickness of the plume dictate the actual exposure level. For instance, a thin, high-altitude haze blocks less UV than a dense, low-hanging plume. Relying on smoke as a form of sun protection is ill-advised, and prolonged outdoor exposure under smoky skies still results in cumulative UV exposure and potential skin damage.
Non-UV Health Concerns of Smoky Air
Beyond the question of tanning, exposure to heavy smoke carries immediate health risks unrelated to UV radiation. The primary concern is the inhalation of fine particulate matter, specifically particles smaller than 2.5 micrometers (PM2.5). These particles are small enough to bypass the body’s natural defenses and lodge deep within the lungs and enter the bloodstream.
Inhaling PM2.5 can cause burning eyes, a runny nose, and throat irritation in healthy individuals. It can also aggravate pre-existing respiratory conditions like asthma and chronic obstructive pulmonary disease (COPD), leading to wheezing, coughing, and shortness of breath. The toxic mixture also includes harmful gases like carbon monoxide and volatile organic compounds, which can impact the cardiovascular system.
Health officials advise limiting outdoor activity when air quality is poor, regardless of the reduced UV levels. The risk to respiratory and cardiovascular health far outweighs any perceived benefit of a reduced chance of sunburn.