*Study Limitations*

It should be noted that a limitation of the study was the di fficulty in grading both sunlight exposure and strain by manual work. The method chosen (giving general examples for each grade level) was quite vague, and several patients did not fully understand how to self-assess themselves. However, a more quantitative categorization (e.g., estimated intensity combined with mean daily hours of sun light exposure or manual work) proved to be impractical or more complicated based on discussions with patients during the study preparation stage. Additionally, exposition to the discussed factors did not occur under controlled conditions. Other unknown factors might have played a role, too, so that uncertainties in the hypothesized chain of causation remain.

**Author Contributions:** Conceptualization, U.G.; methodology, U.G.; software, U.G.; F.D.; validation, F.D., R.L.; U.G.; formal analysis, U.G.; H.-L.N.; R.L.; F.D.; investigation, U.G.; F.D.; data curation, U.G.; F.D.; writing—original draft preparation, U.G.; H.-L.N.; F.D.; writing—review and editing, U.G.; H.-L.N.; R.L.; S.M.; K.T.; F.D.; visualization, U.G.; F.D.; supervision, U.G., F.D.; project administration, U.G.; F.D.; All authors have read and agreed to the published version of the manuscript.

**Funding:** This research did not receive any specific gran<sup>t</sup> from funding agencies in the public, commercial, or not-for-profit sectors.

**Acknowledgments:** We are indebted to the HHT patients for their participation in the study, and the physician assistant Ms. Monika Hentschel for her help in data acquisition. Part of this study was presented at the IX. International Scientific HHT meeting in Kemer, Turkey, 20.-24.5.2011, the annual meeting of the German society for otorhinolaryngology 1.-5.6.2011 and has been published as abstract in Hematology Reports 2011; 3(s2): 33. We acknowledge support by the Open Access Publication Fund of the University of Duisburg-Essen.

**Conflicts of Interest:** Urban Geistho ff is consulting and collaborating with patient organizations for HHT (www.hht.org, www.morbus-osler.de). There are no other relevant conflicts of interest or disclosures.
