Several authors have commented about worsening of prurigo pigmentosa in the spring and summer,^{1,3,8,9,11,31,32,45,51,66,67,74} but those statements are anecdotal. Sunlight was thought to be influential,^8,21 but testing for photosensitivity failed, consistently, to confirm that impression. Nagashima et al. were the first to suggest that sweating could provoke an eruption of prurigo pigmentosa,^1,3 but that thesis has yet to be proved. Friction from clothing also has been claimed to worsen matters.^3,79 Nagashima, in 1978, proposed that "friction from clothing may act not as a contact allergen but as a nonspecific mechanical stimulus"; that, too, remains speculation.

H. Pinkus thought that "new environmental factors" might be responsible for prurigo pigmentosa.¹⁰⁶ None, though, have been identified as truly being causative. Other students of the subject have suggested that prurigo pigmentosa could be akin to pigmented contact dermatitis caused by the optical whitener, Tinopal (a combination of 1-(3-chlorphenyl)-3-phenyl-pyrazoline and 1-(3-chlorphenyl)-3-(4-chlorphenyl)-pyrazoline).^3,107 Comprehensive patch tests, including to optical whiteners, were carried out in 16 patients with prurigo pigmentosa,^{1,3,6,11,1953,108} but only in two patients were there positive reactions to chromium,^11,108 an indication that the disease is not related to contact allergens.

Although Nagashima, in 1978, noted "no systemic changes" in his patients with prurigo pigmentosa,^1,3 reports recently, including one of Teraki and Nagashima et al.,⁶² claimed that an association existed between nutrition and prurigo pigmentosa. Note has been taken of eruptions of prurigo pigmentosa that seemed to coincide with dieting, weight loss, anorexia nervosa,⁵³ and implementation of vegetarian cuisine.⁷⁶ Diabetes mellitus also has been implicated as a factor. Thirty patients with prurigo pigmentosa were said to have ketonuria, either as a consequence of diabetes mellitus^{35,62,75,83–86,88} or of dieting.^{34,41,43,53,62,72–74,80,81,90} Teraki et al. stressed that while "The eruption [of prurigo pigmentosa] cleared when the ketosis diminished", serum levels of glucose seemed to have no influence on the disease.⁶² Nutrition appeared to improve the condition in patients who dieted, and insulin was said to expedite regression of lesions in patients with diabetes. Most patients with prurigo pigmentosa, however, do not have ketosis or diabetes. No mention was made ever about any disorders of the gastrointestinal tract in patients with prurigo pigmentosa, unlike the situation in dermatitis herpetiformis, where an association with glutensensitive enteropathy is established. A few reports tell of a relationship between prurigo pigmentosa and other diseases, but those seem to be coincidental.^{1,3,6,15,23,87}

In sum, friction, heat, sunlight, sweating, and allergic contact sensitization have been said to be connected in some way to induction of prurigo pigmentosa, as have been diabetes mellitus and ketonemia. No compelling evidence exists on behalf of any of these propositions.