Shomer Peta'im Hashem - God Watches Over the Unwise
One can't help but notice how so many people routinely engage in dangerous activities without thinking twice. This conduct seems to contradict the Torah's prohibition against engaging in anything which may be damaging to one's health. Indeed, we are required to avoid dangerous activities even more so than ritual prohibitions. Whether it be smoking, air travel, skiing, riding roller coasters, and even speeding when driving, such activities seem to be tolerated, if not outright acceptable. How is it permissible for a Jew to do such things?
The justification for engaging in dangerous activities is based on a halachic concept known as "shomer peta'im Hashem" which translates as, "God guards the unwise." According to this principle, something which is considered to be a mainstream or routine activity is permitted to be performed even though it includes dangerous elements. This is true as long as the risks involved are proven to be negligible. It is explained that the prohibition against engaging in dangerous activities does not include those things which most people routinely engage in. As such, one may engage in all recreational sports, pastimes, and other pursuits even though they may not be completely safe, as long as they are considered normative activities. Although air travel poses a number of significant risks, one is permitted to fly since it has become a standard method of transportation despite the dangers involved.
The principle of shomer peta'im Hashem pertains exclusively to situations where the danger is minimal and disaster only occurs in a negligible number of cases. Nevertheless, it is permitted to engage in slightly higher-than-normal risks if doing so is required as part of one's livelihood or work responsibilities. Even so, any act which poses a direct and immediate risk to one's life is always forbidden. God does not provide any Divine protection whatsoever for those who intentionally engage in activities which are considered by most people to be extreme or exceptionally risky.
An unfortunately all too common habit which many people engage in is smoking. Although one may be led to believe otherwise, smoking is completely forbidden according to Torah law, as it is harmful to one's health.  While it is absolutely forbidden for one to take up smoking, it is unclear, however, if those who currently smoke are required to immediately quit. This is especially true regarding those who started smoking many years ago before it was revealed how dangerous the habit truly is. Some authorities argue that those who are genuinely unable to quit smoking qualify under the clause of shomer peta'im Hashem despite the health risks involved. In addition to smoking, the use of any substances which are harmful to the body are likewise forbidden. One is even forbidden to purchase cigarettes on behalf of another person, as doing so is assisting them in harming themselves.
 Devarim 4:15
 Chullin 9b
 Tehillim 116:6
 Shabbat 129b, Yevamot 12b, and Nidda 31b, Avoda Zara 30b
 Aruch Hashulchan O.C. 3:6
 Chelkat Yaakov C.M. 31
 Achiezer 1:23
 Bava Metzia 112b
 Binyan Tzion 1:137
 Kovetz Shiurim;Ketubot 136
 Rivevot Ephraim 8:586, Tzitz Eliezer 15:39, Aseh Lecha Rav 2:1
 Igrot Moshe C.M. 2:76
 Bava Kama 92a
 Aseh Lecha Rav 6:58, 7:65