Exogenous factors capable of inducing basal-cell carcinoma:

 
Radiation therapy (XRT):
 
"These results largely agree with those of previous studies on the risk of BCC in relation to ionizing radiation exposure." (Lichter, 2000) [42]
 
Arsenic exposure:
 
"The most common arsenic-induced skin cancers are Bowen's disease (carcinoma in situ), basal cell carcinoma (BCC) and squamous cell carcinoma (SCC). Arsenic-induced Bowen's disease (As-BD) is able to transform into invasive BCC and SCC." (Yu HS, 2006) [43]
 
Chemical carcinogens:
 
"Other exogenous factors include . . . hydrocarbons and industrial oils (e.g. petrolatum products, coal)." (Mehrany, 2007) [8]