Ultraviolet is part of the electromagnetic spectrum with wavelengths between 100 and 400 nm, which is between visible light and x-rays (1, 2). According to the wavelength, ultraviolet is divided into ultraviolet A (UVA) (320–400 nm), ultraviolet B (UVB) (280–320 nm), and ultraviolet C (UVC) (100–280 nm) (3, 4). UV radiation is a mutagenic agent, and long-term overexposure to sunlight has been linked to photoaging and the development of skin cancer. Thus, it is considered one of the most common environmental factors that damage the skin's structure and function (5, 6). Different UV wavelengths cause skin damage through different mechanisms (7). UVA has a strong penetration ability into the skin through the cuticle, the epidermis, the dermis, and even subcutaneous tissue. The UVB penetrating ability of the skin is weak, which mainly causes damage to the epidermis and the superficial dermis (2, 8, 9). However, as the energy produced by ultraviolet decreases with increasing wavelength, UVB has a greater damaging effect on the epidermis than UVA (10, 11).

Erythema is the most apparent feature of ultraviolet radiation on the skin (12, 13, 47). It appears in the form of a visible redness of the skin that results from an increase in the blood volume of the superficial and deep dermal vessels for at least 6 h after UV exposure (12). This marked dilation of blood vessels can induce the formation of edema and the accumulation of white blood cells as an inflammatory response. At present, several studies have reported that ultraviolet irradiation, mainly UVB irradiation, can cause an increase in the local blood flow (12, 14–16, 46). Clinically, minimal erythema dose (MED) is usually used to evaluate skin sensitivity to ultraviolet radiation. MED refers to the minimum dose (J/m²) or the shortest time (s) in which erythema becomes visible on the skin 24 h after irradiation (17). While UVA is not primarily responsible for erythema, it does play a significant role in pigmentation (11, 18, 19). Practically, both UVA and UVB are used in tandem, not UVB alone, to induce skin erythema. Therefore, the role of UVA in erythema induction merits further discussion.

Optical coherence tomography (OCT) is a non-invasive diagnostic technique with an effective scanning depth of 2 mm (20). The technique is similar to ultrasound scanning, but OCT has a much higher resolution (< 10.0 um) at the same imaging depth because it is near-infrared (21). Dynamic OCT, also called speckle variance OCT or optical microangiography, is based on optical Doppler tomography, which combines Doppler velocimetry and optical coherence tomography to measure the movement velocity of blood cells at discrete spatial positions in skin tissue, and can capture microvascular blood flow with a blood flow rate as low as 20 mm/s (20, 22–24). Therefore, dynamic OCT enables the visualization and measurement of blood vessel morphology, providing a new perspective on skin health, inflammation, and tumor lesions. Dynamic OCT combines conventional OCT imaging with blood flow data to produce a living image of skin microvessels. Significantly, dynamic optical coherence tomography (OCT) has demonstrated its immense value in characterizing inflammatory processes and discerning the functional and metabolic attributes of various skin diseases (25). A number of studies have employed dynamic OCT to scrutinize the microvascular architecture and changes in blood flow within acne lesions. These studies indicate an augmented vascular signal proximal to comedones, as observed in the dynamic OCT en-face view (26). Intriguingly, the vascular signal in the apparently unaffected skin of acne patients was found to be not significantly different from that in the skin of healthy volunteers without acne (25).

The present study used UVB and UVA + UVB to irradiate skin to induce erythema and then detected the changes in the blood flow at the MED erythema caused by UVB and UVA + UVB radiation through OCT to explore the role of two different UV lights in causing skin erythema.

While UV radiation is crucial for life, excessive exposure, particularly to UVB, can precipitate various health concerns, including DNA and protein structural damage and premature skin aging (29). Acute damage from UVB irradiation can present as sunburn, photoaging, and melanoma, each of which carries significant health risks. Due to UVB's high intensity and short wavelength, it causes considerable damage to the epidermis (30). UVB is also categorized as carcinogenic because of its potential to induce skin cancer after prolonged exposure (31). Erythema represents the skin's initial inflammatory response to UV irradiation.

Consequently, in clinical diagnosis and research, ultraviolet lights are usually used to induce erythema, and MED is used to carry out studies and investigate skin-related diseases. Justus et al. proved that, in healthy subjects whose forearm skin was irradiated with ultraviolet light with a wavelength of 270–380 mn in a 3 cm-diameter area, the blood flow increased in a dose-dependent manner and peaked 12 and 36 h after irradiation, respectively (12). Other studies have also confirmed that acute UV exposure causes an increase in blood flow to the skin (14, 32, 33). UVA primarily induces immediate pigment darkening (IPD) and persistent pigment darkening (PPD), while UVB predominantly causes delayed tanning (DT), which is often preceded by erythema. Therefore, UVB is more erythemogenic than UVA (11, 15, 18, 19). Although erythema is typically triggered by UVB, a combined UVA and UVB radiation source is usually employed in practice to induce the erythema irritation model. As such, the role of UVA in the process of erythema induction is certainly worth discussing. Considering the above reasons, we did not set UVA irradiation alone in this study. In this study, skin erythema was induced using UVA + UVB or UVB alone, and the differences in the erythema induced by UVA + UVB and UVB were compared under the same UVB energy. The results showed that skin erythema caused by UVA + UVB was weaker than those caused by UVB alone. The analysis of local blood flow by OCT showed that the peak and maximum depth of local blood flow caused by UVB alone were significantly higher than those caused by UVA+UVB. The analyzed results are shown in Figure 3.

As a non-invasive optical technology, OCT is widely used to detect changes in skin blood flow, including skin damage caused by ultraviolet light. The blood flow measured by OCT depends on the number of red blood cells in the field of vision and the average speed of red blood cells (34). As an established and mature microvascular assessment method, OCT enables the rapid, non-invasive analysis of blood flow patterns in the skin, thus providing continuous and accurate measurements (34–36). Current studies have also demonstrated that OCT is an effective method for detecting, locating, and quantifying changes in skin blood flow, especially for assessing changes in microvascular blood flow (37). Ou Qin et al. utilized OCT to evaluate changes in blood flow following treatments with sodium lauryl sulfate and tape stripping on the skin. They observed a notable increase in blood flow volume in the sodium lauryl sulfate group 1 day after the intervention compared to the distilled water group. This elevated blood flow volume subsequently declined gradually, reverting to the levels observed in the control group by the third-day post-intervention.

Conversely, tape stripping did not impact blood flow volume from Day 1 to Day 3 (38). OCT is sensitive to the movement of blood cells, thus providing complete information about the shape of blood vessels and blood flow data that reaches the maximum depth of the skin in the middle dermis (21, 39, 45). OCT has a good correlation and repeatability in assessing changes in skin blood flow (40, 41) and has the advantages of locating the dermal-epidermal junction and quantifying the size and distribution of blood vessels (40). In this study, OCT was used to detect the blood flow distribution in the MED erythema area, which measured the maximum subcutaneous depth of blood flow and calculated the total and the peak of blood flow in the test areas. These parameters can objectively evaluate the severity of skin erythema.

Although UVB is the main ultraviolet band causing skin erythema, UVA + UVB is usually used to induce MED. Our study found that the intensity of erythema induced by UVA + UVB was weaker than that induced by UVB. Under the same total amount of local subcutaneous blood perfusion, UVA + UVB could reduce the peak value of local blood flow and cause a shallow abnormal maximum depth of subcutaneous blood flow. Therefore, UVA + UVB is safer in reducing the degree of skin injury than UVB. We believe that UVA has strong penetration and can reach the dermis, thereby activating macrophages and plasma cells in the subcutaneous tissue to participate in the regulation of immune response; UVA simultaneously promotes the expression of matrix metalloproteinase on dermis fibro-cells, reducing inflammatory cell infiltration and inflammatory response (42). Several studies have reported that UVA exposure can confer a degree of photoimmune protection against immunosuppression, with the photoimmune effect being contingent on the UVA dosage (37, 43). Reeve and his colleagues proposed that UVA radiation is not only immunologically benign but also generates photoproducts capable of counteracting radiation-induced immunosuppression. They suggested that, at least in mice, UVA radiation might exert a photoprotective effect if administered prior to UVB exposure (43). Furthermore, UVA radiation has been demonstrated to trigger an increase in the expression of heme oxygenase (HO), a surge that persisted for at least 3 days post-UVA exposure. This induction of HO can forestall UVB-induced immunosuppression by curbing oxidative stress (44). Our findings lend additional support to the concept of photoimmune protection against UVB. Therefore, when using ultraviolet light to induce skin erythema, using UVA + UVB as the light source can reduce skin inflammation and local blood flow peak and avoid strong skin erythema reactions, thus reducing skin damage.

The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding authors.

The studies involving human participants were reviewed and approved by the Ethics Committee of Skin Disease Hospital of Tongji University. The patients/participants provided their written informed consent to participate in this study.

G-BF and J-WY designed the experiment and wrote the manuscript. FT, H-MK, and QL are the main members of the experiment, they conducted the experiment, and analyzed the data. G-BF, YZ, and Y-MT designed the experiment and edited the manuscript. All authors contributed to the article and approved the submitted version.