Hydroxyl (OH) radicals remove many man-made and natural gases from the atmosphere and therefore play a key role in global tropospheric chemistry. Recent increases in CH4 and CO have caused concern that the levels of OH may decrease, thus reducing the capacity of the atmosphere to remove and control man-made pollutants. The authors have modeled OH concentrations over a wide range of climatic conditions, to examine its long term stability and to determine the major factors causing changes in its levels. The authors used a one dimensional photochemical model based on measurements of CH4 and N2O from polar ice cores and current understanding of the sources and sinks of CO, H2, and NOY. The authors find that mean OH concentrations are strongly buffered against changes in the chemical and climatic state of the atmosphere. Their model calculations indicate that only a small part of the factor of five difference observed in methane concentrations between the present and the peak of the last ice age is due to changes in OH levels. It also appears that in the last 100-200 years human activities have more than doubled the atmospheric concentrations of CH4 and CO.