Electron beam deposition (EBD) provides an inexpensive way to fabricate nanostructures of various materials in a scanning electron microscope (SEM). However, the purity of metals deposited from an organometallic precursor gas is impaired by simultaneously deposited carbon coming both from the organometallic molecule and the residual contamination gas in the SEM chamber. We discuss carbon-contamination EBD in a standard high-vacuum SEM and compare it to EBD of Au in an environmental SEM (ESEM). The ESEM allowed us to perform "environmental" EBD (E-EBD), i.e., EBD in the presence of an environmental gas (1-10 Torr) in addition to the organometallic precursor gas. We built a simple device that contains a reservoir for the organometallic precursor and goes on the sample stage of the ESEM. With this device we were able to highlight the advantages of E-EBD over conventional, high-vacuum EBD. We discuss the basic chemical reactions underlying the E-EBD process.