How can one distinguish between the different types of redness?

The acne most often appears in oily skin, due to an overproduction of sebum by the sebaceous glands. This excess sebum clogs pores and creates an environment favorable to the proliferation of Cutibacterium acnes, a bacterium naturally present on the skin but which becomes pathogenic when it multiplies. By feeding on the triglycerides contained in sebum, C. acnes releases pro-inflammatory fatty acids that irritate the walls of the hair follicles. This mechanism then triggers a local inflammation, explaining the appearance of redness around the pimples.

When inflammatory acne develops, the body triggers an immune response. Neutrophils and macrophages are recruited to the affected area, while chemical mediators such as interleukins and prostaglandins are released into the skin. These pro-inflammatory molecules lead to a vasodilation of the blood capillaries, meaning an increase in their diameter to deliver more blood, oxygen, and nutrients to the tissues. This process intensifies redness, making blemishes more visible, particularly on fair skin.

Even after blemishes disappear, some redness can persist: this is known as post-inflammatory erythema (PIE). These flat pink-to-red spots appear where the lesions once were. They result from residual dilation of capillaries rather than excess melanin, unlike post-inflammatory hyperpigmentation, which more often affects darker skin types. Although redness from past imperfections is not classified as scarring, it can still disrupt an even skin tone and impact self-confidence.

Post-inflammatory erythemas generally fade on their own, but their resolution can be prolonged, lasting anywhere from a few months to several years, depending on the phototype, the intensity of the initial inflammation and sun exposure habits. Indeed, UV radiation stimulates the production of free radicals and slows skin regeneration, which tends to prolong the persistence of redness. This is one of the reasons why a daily sun protection is essential, particularly for those prone to redness.

Redness resulting from the dilation of blood capillaries primarily presents as couperose or rosacea, chronic conditions that mainly affect the central area of the face (cheeks, nose, forehead, chin). It should be noted that couperose corresponds to the vascular form of rosacea, the most common form, in which the dermal capillaries dilate excessively, leading to visible redness and sometimes sensations of warmth.

These episodes of redness are particularly common in sensitive skin, where the skin barrier is compromised and the sensory fibers are highly reactive. They can be temporary, resolving within minutes or hours (referred to as erythema), or persist for days, even weeks (in which case it is termed erythrosis). Although more common in fair skin phototypes, all skin tones can be affected by rosacea. The exact causes of rosacea remain only partially understood, but several biological and environmental factors appear to promote blood vessel dilation:

• Genetics : Up to 30% of people with rosacea have a family history, suggesting a hereditary component. Certain receptors involved in inflammatory responses may be more prevalent in these individuals.

• Colonization by Demodex : When they proliferate, these tiny parasites naturally present on the skin can secrete proteases that activate epidermal PAR-2 receptors. This activation triggers the release of pro-inflammatory cytokines, such as TNF-α and IL-1, which are responsible for inflammation and redness.

• Heat and consumption of spicy foods : The stimulation of thermosensitive TRPV1 receptors by heat or by compounds such as capsaicin, found in spicy foods, triggers local vasodilation and intensifies hot flashes and flushing.

• Stress : Stress can activate TRPA1 and TRPV1 receptors, triggering the release of vasodilatory neuropeptides such as CGRP and PACAP. These molecules promote capillary dilation and amplify erythema. Cortisol may also play a role in this process, although the exact mechanism remains unknown.

• Exposure to UVB : UV rays penetrate the epidermis and activate TRPV4 receptors, contributing to the breakdown of the skin’s structural integrity and to vasodilation, thereby intensifying redness.

Note : The rosacea primarily affects women and most often manifests in individuals in their thirties.

Redness can also occur following skin exposure to an allergen or irritant, triggering what is referred to as a contact eczema, or contact dermatitis. Unlike atopic eczema, this condition is not linked to a genetic predisposition to allergies. It is an acquired inflammatory reaction of the skin, which can manifest even in individuals without any atopic history. The allergens responsible for irritation-induced redness can originate from a variety of sources: clothing, adhesives, nickel-containing jewelry, cosmetics, medications, cement, paints, pesticides...

Contact dermatitis evolves in two phases. The first is the sensitization phase, during which the allergen penetrates the skin and binds to skin proteins to form an antigenic complex. T cells of the immune system recognize this complex and retain a memory of the allergen without immediately causing visible symptoms. This phase can last from several days to several weeks.

Upon re-exposure to the allergen, activated lymphocytes release lymphokines and other inflammatory mediators. This is the so-called elicitation phase. This response leads to characteristic symptoms: erythema, pruritus, edema, vesicles, desquamation, skin thickening… Lesions usually appear at the site of contact but may, in severe cases, extend over large areas of skin.

Irritation-induced redness typically resolves within one to two weeks after the responsible allergen is eliminated.

• YOUN S. W. & al. Acne erythema improvement by long‐pulsed 595‐nm pulsed‐dye laser treatment: A pilot study. Journal of Dermatological Treatment (2008).

• EMER J. & al. Update on the management of rosacea. Clinical, Cosmetic and Investigational Dermatology (2015).

• BUCKLEY D. & al. The red face. Textbook of Primary Care Dermatology (2021).

• MONTERO-VILCHEZ T. & al. Development of an emulgel for the effective treatment of atopic dermatitis: Biocompatibility and clinical investigation. Gels (2024).

• HAKOZAKI T. & al. Skin inflammatory signatures, as measured by disordered spatial redness patterns, predict current and future skin ageing attributes. Experimental Dermatology (2024).