Family history is a risk factor for asthma, with many different genes being implicated.  If one identical twin is affected, the probability of the other having the disease is approximately 25%.  By the end of 2005, 25 genes had been associated with asthma in six or more separate populations, including GSTM1 , IL10 , CTLA-4 , SPINK5 , LTC4S , IL4R and ADAM33 , among others.  Many of these genes are related to the immune system or modulating inflammation. Even among this list of genes supported by highly replicated studies, results have not been consistent among all populations tested.  In 2006 over 100 genes were associated with asthma in one genetic association study alone;  more continue to be found. 
Climate change is also being studied as a factor in the increased incidence of asthma. Maternal smoking during pregnancy is a risk factor for asthma and poor outcomes. Tobacco smoke is also a significant risk factor for the development and progression of asthma. Treatment of environmental allergies with allergen immunotherapy, or allergy shots, has been shown to decrease a child's risk of developing asthma. The development of asthma is ultimately a complex process influenced by many environmental and genetic factors, and currently there is no proven way to decrease an individual's risk of developing asthma.