## 2 Nitrogen Dioxide Air Quality Standards
\begin{table}
\begin{tabular}{|c c c c c c|} \hline \hline
**Ref.** & **Location** & **Environment** & **Fuels** & **NO\({}_{2}\) Concentration** & & **Exposure Period** \\  & & & & **Indoors** & **Outdoors** & \\ \hline
[28] & Hong Kong & Urban (12) & Town gas & Summer: & & 1 week \\  & & & & 61.0 \(\pm\) 22.4 ug\(\cdot\)m\({}^{-3}\) & 71.8 \(\pm\) 15.1 ug\(\cdot\)m\({}^{-3}\) & \\ \hline
[18] & Netherlands & & NG, Electricity (5\%) & Winter: & & 1 week \\  & Urban (299) & & 96 \(\pm\) 59 ug\(\cdot\)m\({}^{-3}\) & 62 \(\pm\) 12 ug\(\cdot\)m\({}^{-3}\) & \\  & Rural (149) & & 64 \(\pm\) 50 ug\(\cdot\)m\({}^{-3}\) & 35 \(\pm\) 19 ug\(\cdot\)m\({}^{-3}\) & \\  & Rural (164) & & 75 \(\pm\) 62 ug\(\cdot\)m\({}^{-3}\) & 22 \(\pm\) 9 ug\(\cdot\)m\({}^{-3}\) & \\  & Urban & & Unvernted geiser (181): & & & \\  & & & 120 \(\pm\) 55 ug\(\cdot\)m\({}^{-3}\) & & & \\  & & & Vented geiser (48): & & & \\  & & & & 71 \(\pm\) 53 ug\(\cdot\)m\({}^{-3}\) & & \\  & & & & No geiser (44): & & \\  & & & & 49 \(\pm\) 30 ug\(\cdot\)m\({}^{-3}\) & & \\  & Rural & & Unvernted geiser (181): & & & \\  & & & 143 \(\pm\) 60 ug\(\cdot\)m\({}^{-3}\) & & & \\  & & & & Vented geiser (105): & & \\  & & & & 60 \(\pm\) 38 ug\(\cdot\)m\({}^{-3}\) & & \\  & & & & No geiser (34): & & \\  & & & & 32 \(\pm\) 24 ug\(\cdot\)m\({}^{-3}\) & & \\ \hline
[19] & Portage & Rural & NG (2:37, O:240) & 65.5 \(\pm\) 30.7 ug\(\cdot\)m\({}^{-3}\) & 15.8 \(\pm\) 6.3 ug\(\cdot\)m\({}^{-3}\) & 1-week \\  & & LPG (1:568, O: 555) & 65.6 \(\pm\) 38.4 ug\(\cdot\)m\({}^{-3}\) & 11.8 \(\pm\) 5.6 ug\(\cdot\)m\({}^{-3}\) & \\  & & Electric (1:174, O:173) & 8.4 \(\pm\) 4.7 ug\(\cdot\)m\({}^{-3}\) & 12.8 \(\pm\) 5.6 ug\(\cdot\)m\({}^{-3}\) & & \\ \hline
[35] & Boston & Urban & Gas & Heating season (57): & & 2-weeks \\  & & & 94 \(\pm\) 36 ug\(\cdot\)m\({}^{-3}\) & 39 \(\pm\) 11 ug\(\cdot\)m\({}^{-3}\) & & \\  & & & Non heating season (43): & & 62 \(\pm\) 34 ug\(\cdot\)m\({}^{-3}\) & 32 \(\pm\) 13 ug\(\cdot\)m\({}^{-3}\) & \\  & & & Total (100): & & & \\  & & & 81 \(\pm\) 38 ug\(\cdot\)m\({}^{-3}\) & 36 \(\pm\) 12 ug\(\cdot\)m\({}^{-3}\) & & \\ \hline \hline \end{tabular}  
* Fuels : NG: natural gas; LPG: liquefied petroleum gas.  
\end{table} 
Table 2: _Cont._some countries and organizations have defined stricter limits than those set by the WHO. For instance, Australia significantly strengthened NO2 reporting standards for the 1-h and annual average to 150 \(\upmu\)g\(\cdot\)m\({}^{-3}\) and 28.2 \(\upmu\)g\(\cdot\)m\({}^{-3}\), respectively, bringing forward standards initially proposed for 2025 [42; 43]. Similarly, the National Institute of Public Health from Norway set guidelines for the 1-hour and annual average NO2 limits of 100 \(\upmu\)g\(\cdot\)m\({}^{-3}\) and 30 \(\upmu\)g\(\cdot\)m\({}^{-3}\), respectively [44]. As mentioned above, although indoor sources play an important role in NO2 exposure, the EU as many countries, including the U.S., do not have indoor standards for protecting the general population from NO2 exposure [2]. Exceptions include countries like Canada and China. Canada, for instance, has defined a value of 170 \(\upmu\)g\(\cdot\)m\({}^{-3}\) for short-term exposures and 20 \(\upmu\)g\(\cdot\)m\({}^{-3}\) for long-term exposures, which are stricter than those set by the WHO levels. Table 3 provides an overview of the air quality NO2 guidelines and standards for various countries and organizations over time.