The principle of cumulative damage was established in the early eighties. When individuals with normal skin are exposed to repeated suberythemal doses of UVA or UVB, they will develop erythema within five days [34
]. This implies that the damage induced by low-dose UV accumulates with time and eventually becomes clinically apparent. These studies also demonstrated that chronic exposure to low-dose UV sensitizes the skin, such that the MED decreases in a time-dependent fashion [34
]. Significant cellular changes take place in response to chronic suberythemal UV exposure, including epidermal hyperplasia, stratum corneum thickening, depletion of Langerhans cells, increased dermal inflammatory infiltrate, and deposition of lysozyme on elastin fibers [37
These studies also indicated, however, that damage will only accumulate when the daily UV dose exceeds a specific threshold— if the irradiance is too miniscule, erythema will not develop, even after repeated daily exposures. For UVA (320–410 nm), the threshold dose was 0.15 MED (3.8 J/cm2
), for UVB (270–320) it was 0.25 MED (4.7 mJ/cm2
), and for UVC it was 0.50 MED (6.5 mJ/cm2
Damage accumulates when the skin is not given adequate time to recover from the initial insult. After irradiation with 0.75 MED, it takes 30–48 hours to recover from UVA and 24–30 hours to recover from UVB [38
]. If repeated exposures are spaced appropriately, the skin will recover and erythema will not develop. Unfortunately, this is not a practical solution for the average patient, who is exposed to light bulbs on a daily, if not hourly, basis ().
Landmark papers establishing the principle of cumulative damage
These studies established the foundation for the concern that patients might be at risk from cumulative, low-dose exposure to light bulbs. However, these results might underestimate the risk posed to lupus patients for two reasons. The first is that subjects used in these studies had normal skin. It is likely that photosensitive lupus patients would have a more robust response to lower levels of UVR and may take longer to recover. The second is that these studies lasted a maximum of nine days, while lupus patients are exposed to bulbs for years. It is possible that the threshold doses listed above are capable inducing erythema after a longer period of time. While these studies provide a nice framework, additional work still must be done to understand the true risk to photosensitive patients.