This dissertation includes three chapters. The first and second chapters are related to economics of immigration, and the last one is about environmental economics. The first chapter studies people who work and live in the US on work visas such as H-1B, and compares them to natives. In this chapter I examine whether or not there exists any wage premium for or against college graduate immigrants who are on work visa compared to college graduate natives. I also check for any change of such a premium from 2003 to 2010. On the contrary to the common belief that foreign workers are cheap labor force, my results show that skilled immigrants holding temporary work visas on average have a significant wage premium over natives, and this premium has even increased significantly from 2003 to 2010 (from 14% to 22%). My results show that such a wage premium is different for men, women, and countries of origin, but I found no evidence supporting different premiums across different fields of study.

The second chapter of this dissertation studies the dynamics of the earnings gap between those immigrants and US-born individuals with bachelor's degrees or higher in science and engineering fields. The research question is that in case a gap exists for or against immigrants, how is it changing with the amount of time immigrants reside in the United States? I employ cross-sectional and longitudinal approaches to answer this question, and study the earnings gap between three groups of immigrants (based on the current residency status) and natives at entry and over time. I also compare natives with immigrants who migrated to the United States on different types of visas (permanent residence visa, work visa, study visa, and dependent visa). Results show that, upon arrival, immigrants on average have a considerable premium over the US-born, and this gap, surprisingly, even gets bigger with an approximate rate of 0.25% for the first 5-10 years of immigrants' residence in the US. This phenomenon could be due to the higher level of abilities and motivation among immigrants compared to natives. Another reason can be the selectivity among immigrants, meaning that more successful stays and others return. Unfortunately, due to the lack of information in data regarding these issues, they could not be controlled for in my models.

The last chapter is about environmental economics. This chapter exploits a daily time series data on pollen count and PM2:5 level from 2009 to 2015 to study the separate impacts of PM2:5 and pollen on the number of total, in-patient, and out-patient respiratory hospital admissions within different age groups in the Reno/Sparks metropolitan area in Northern Nevada. The results show that although PM2:5 has a positive impact on the number of out- patient admissions in most of the age groups, there is no evidence that shows any relationship between the pollen count and the number of in-patient or out-patient respiratory admissions.