Muhammad Husain As-Saghir writes:

سَـيِّـدُنَـا الْـمَـرْجِـعُ الْأَعْـلَـى لِـلْـمُـسْـلِـمِـيـنَ عَـلِـيٌّ الـسِّـيـسْـتَـانِـيُّ مُـدَّ ظِـلُّـهُ مُـدَخِّـنٌ مُـقِـلٌّ وَلَـكِـنَّـهُ لَا يُـدَخِّـنُ أَمَـامَ أَحَـدٍ قَـطُّ حَـتَّـى لَـوِ اجْـتَـمَـعْـنَـا مُـنْـفَـرِدَيْـنِ فَـأَنَـا أُدَخِّـنُ وَهُـوَ لَا يُـدَخِّـنُ وَإِنَّـمَـا يُـدَخِّـنُ إِذَا خَـلَا بِـنَـفْـسِـهِ وَحْـدَهُ وَقَـدْ لَا يَـعْـلَـمُ أَحَـدٌ فِـي الْـحَـوْزَةِ الْـعِـلْـمِـيَّـةِ فِـي الـنَّـجَـفِ الْأَشْـرَفِ الَّـتِـي هُـوَ زَعِـيـمُـهَـا الْـيَـوْمَ أَنَّ الـسَّـيِّـدَ مِـنَ الْـمُـدَخِّـنِـيـنَ

,,Our Sayyid, the highest model of emulation for Muslims, Ali As-Sistani, may his existence last long, is a moderate smoker. However, he never smokes in front of anyone. Even when the two of us get together alone, I smoke while he does not and he smokes merely when he is completely alone and possibly no one at the theological college in Holy Najaf, of which he is the head today, knows that the Sayyid is one of the smokers.” [Fiqh-ul-Hadarah, page 42 – 43]

Comment:

One should be knowledgeable about this relevant information.