Introduction: Increased environmental pollutants caused by industrial development, population, urban construction, the increased use of synthetic materials and chemicals as well as climate change have, in recent decades, led to air pollution becoming an important issue in the field of conservation of cultural property for scientists and conservation and restoration expertsin the world. Furthermore, the role of air quality in the preservation of historical objects and art works in museums has been one of the fundamental topics of the preventive conservation of museum collections. However, considering adverse effects such as corrosion, reduced fibre strength and changing colour due to gaseous pollutants emitted by traffic and industry, NO2 and SO2 levels were monitored and assessed in the Library and Malek National Museum and in the Museum of Reza Abbasi in the centre and North of Tehran.Materials and Methods: Sampling of the aforementioned pollutants using Palmes passive sampler tubes was carried out in exhibition halls, display cases and storage areas in the museums, twice in the summer and winter of 2012-2013. For this purpose, three tubes were used to take samples from each location. One of the tubes as a reference was completely closed. Nitrogen dioxide was absorbed by TEA:Triethanolamine and measured using ultraviolet spectrophotometry (Byanjuet al. 2012). The concentration of sulphur dioxide was measured by using ion chromatography with KOH used as an adsorbent (Lopez-Apariciaet al. 2010). Simultaneously, temperature and relative humidity inside the display cases and exhibition halls were measured by data loggers for a year and at intervals of 15 minutes. Air exchange rate (AER) of the enclosures can be measured via the tracer gas technique over 24-hour periods. In this project, CO2 was used as the tracer gas and Vaisala, GM70, GMP222 as the detector inside the case. Results and Discussion: The results of micro climate measurements show that temperature and relative humidity and their fluctuations in the museum, especially RH, are affected by the external environment. This indicates a much to high air exchange rate with the outside and the lack of an efficient mechanical ventilation and air conditioning system at the museums. Comparison of the results measurements shows that the concentration of pollutants in the Reza Abbasi Museum is more than in the Malek National Museum. The concentration of NO2 is greater in summer than winter and, vice versa, the concentration of SO2 is higher in winter than summer. Also, the concentration of pollutants in both museums is higher than museum standards. Considering the air exchange rate of the display cases and the concentration of gaseous pollutants at the museums, although the display cases are semi-airtight which reduces the concentration of nitrogen dioxide, surprisingly the concentration of sulphur dioxide outside them has no significant difference. It should be noted that, according to the type of objects and display case materials (glass and metal), there are no sources for these pollutants in the display cases.Conclusion: This study shows the situation of the museums in the urban setting and vicinity to major and crowded streets as well as the air exchange rate play an important role in the concentration of air pollutants in these indoor environments. Furthermore the results show that the concentration of SO2 and NO2 in both museums is higher than standards for the conservation of museum collections. Due to adverse effects of the pollutants, implementing a preventive conservation program is recommended and specific recommendations in this regard will be discussed in a separate article.