###### Abstract
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Ref.** & **Location** & **Environment** & **Fuels** & **NO2 Concentration** & & **Exposure Period** \\ \hline
[14] & Pakistan & Urban & NG & Winter (16): & & 1 week \\  & & & & 218 \(\pm\) 12 ug\(\cdot\)m\({}^{-3}\) & 64 ug\(\cdot\)m\({}^{-3}\) & \\  & & Rural & NG & Summer (30): & & \\  & & & & 234 \(\pm\) 196 ug\(\cdot\)m\({}^{-3}\) & 138 ug\(\cdot\)m\({}^{-3}\) & \\  & & & Winter (20): & & 22 ug\(\cdot\)m\({}^{-3}\) & \\  & & & & 242 \(\pm\) 57 ug\(\cdot\)m\({}^{-3}\) & 22 ug\(\cdot\)m\({}^{-3}\) & \\  & & & Summer (16): & & 81 \(\pm\) 65 ug\(\cdot\)m\({}^{-3}\) & 26 ug\(\cdot\)m\({}^{-3}\) & \\  & & Rural & Biomass & Winter (20): & & 256 \(\pm\) 40 ug\(\cdot\)m\({}^{-3}\) & 22 ug\(\cdot\)m\({}^{-3}\) & \\  & & & & & 51 \(\pm\) 27 ug\(\cdot\)m\({}^{-3}\) & 40 ug\(\cdot\)m\({}^{-3}\) & \\ \hline
[17] & Hertfordshire (UK) & Urban & Gas & Winter: & & 1 week \\  & & & & 38.74 \(\pm\) 12.98 ug\(\cdot\)m\({}^{-3}\) & - & \\  & & & & - Non-smokers: & - & \\  & & & & 18.9 ug\(\cdot\)m\({}^{-3}\) & - & \\  & & & & - Passive smokers: & - & \\  & & & & 21.7 ug\(\cdot\)m\({}^{-3}\) & - & \\  & & & & - Smokers: & - & \\  & & & & 24.8 ug\(\cdot\)m\({}^{-3}\) & - & \\  & & & & Summer: & & 26.70 \(\pm\) 2.45 ug\(\cdot\)m\({}^{-3}\) & - \\  & & & & - Non-smokers: & - & \\  & & & & 14.0 ug\(\cdot\)m\({}^{-3}\) & - & \\  & & & & - Passive smokers: & - & \\  & & & & 14.0 ug\(\cdot\)m\({}^{-3}\) & - & \\  & & & & - Smokers: & - & \\  & & & & 16.2 ug\(\cdot\)m\({}^{-3}\) & - & \\  & & & & Winter: & 13.35 \(\pm\) 5.27 ug\(\cdot\)m\({}^{-3}\) & - \\  & & & & Summer: & & 20.69 \(\pm\) 3.20 ug\(\cdot\)m\({}^{-3}\) & - \\ \hline
[10] & Taiwan & & & Winter: & & \\  & Rural (11) & NG (5), LPG (4) & & 64.69 \(\pm\) 17.11 ug\(\cdot\)m\({}^{-3}\) & 75.41 \(\pm\) 18.05 ug\(\cdot\)m\({}^{-3}\) & 1 week \\  & Urban (12) & LPG & 46.01 \(\pm\) 12.41 ug\(\cdot\)m\({}^{-3}\) & 44.19 \(\pm\) 11.85 ug\(\cdot\)m\({}^{-3}\) & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Studies of long-term NO2 exposure (1 week or more) monitoring in kitchens using natural gas (number of samples). \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Ref.** & **Location** & **Environment** & **Fuels** & **NO2 Concentration** & **Exposure Period** \\ \hline
[14] & Pakistan & Urban & NG & Winter (16): & & \\  & & & 218 \(\pm\) 12 ug\(\cdot\)m\({}^{-3}\) & 64 ug\(\cdot\)m\({}^{-3}\) & \\  & & & Summer (30): & & \\  & & Rural & NG & 234 \(\pm\) 196 ug\(\cdot\)m\({}^{-3}\) & 138 ug\(\cdot\)m\({}^{-3}\) & \\  & & & Winter (20): & & 22 \(\pm\) 57 ug\(\cdot\)m\({}^{-3}\) & \\  & & & Summer (16): & & 81 \(\pm\) 65 ug\(\cdot\)m\({}^{-3}\) & 26 ug\(\cdot\)m\({}^{-3}\) & \\  & & Rural & Biomass & Winter (20): & & 256 \(\pm\) 40 ug\(\cdot\)m\({}^{-3}\) & 22 ug\(\cdot\)m\({}^{-3}\) & \\  & & & & Summer (24): & & 51 \(\pm\) 27 ug\(\cdot\)m\({}^{-3}\) & 40 ug\(\cdot\)m\({}^{-3}\) & \\ \hline
[17] & Hertfordshire (UK) & Urban & Gas & Winter: & & 1 week \\  & & & & 38.74 \(\pm\) 12.98 ug\(\cdot\)m\({}^{-3}\) & - & \\  & & & & - Non-smokers: & - & \\  & & & 18.9 ug\(\cdot\)m\({}^{-3}\) & - & \\  & & & - Passive smokers: & - & \\  & & & 21.7 ug\(\cdot\)m\({}^{-3}\) & - & \\  & & & - Smokers: & - & \\  & & & 24.8 ug\(\cdot\)m\({}^{-3}\) & - & \\  & & & Summer: & & 26.70 \(\pm\) 2.45 ug\(\cdot\)m\({}^{-3}\) & - \\  & & & - Non-smokers: & - & \\  & & & 14.0 ug\(\cdot\)m\({}^{-3}\) & - & \\  & & & - Passive smokers: & - & \\  & & & 14.0 ug\(\cdot\)m\({}^{-3}\) & - & \\  & & & - Smokers: & - & \\  & & & 16.2 ug\(\cdot\)m\({}^{-3}\) & - & \\  & & Electric & Winter: & & 13.35 \(\pm\) 5.27 ug\(\cdot\)m\({}^{-3}\) & - \\  & & & & Summer: & & \\  & & & 20.69 \(\pm\) 3.20 ug\(\cdot\)m\({}^{-3}\) & - & \\ \hline
[10] & Taiwan & & & Winter: & & \\  & Rural (11) & NG (5), LPG (4) & & 64.69 \(\pm\) 17.11 ug\(\cdot\)m\({}^{-3}\) & 75.41 \(\pm\) 18.05 ug\(\cdot\)m\({}^{-3}\) & 1 week \\  & Urban (12) & LPG & 46.01 \(\pm\) 12.41 ug\(\cdot\)m\({}^{-3}\) & 44.19 \(\pm\) 11.85 ug\(\cdot\)m\({}^{-3}\) & \\ \hline \hline \end{tabular}
\end{table}