Fluorine
Fluorine is an extremely reactive and highly toxic chemical. It's a gas at room temperature and pressure, and exposure to eyes and other mucous membranes can be dangerous and deadly. The accepted LC-50 -- the level of concentration that will kill 50 percent of animals exposed to it -- is 185 parts per million. Fluorine is a corrosive gas and may react with metal containers.
Chlorine
Chlorine is also a gas at standard conditions of temperature and pressure, and is also potentially very toxic. Industrial accidents and its use as a weapon in World War I have produced a large database of human exposure. Lethal concentration has been observed as low as 430 parts per million for 30 minutes. Chlorine containers may burst when exposed to elevated temperatures. Chlorine can react with and corrode some forms of plastics, rubber and coatings.
Bromine
Bromine is a liquid at standard temperature and pressure, and so storage and transport doesn't involve the same level of hazard as the gaseous halogens. Breathing the vapors can still present a danger, however, albeit a reduced one. Bromine can corrode metals, including iron, steel, stainless steel and copper. Like other halogens, bromine is non-flammable but may accelerate combustion if in the presence of oxygen, a combustible substance, and in sufficient temperature to initiate oxidation.
Iodine
Iodine is considerably less dangerous than the other halogens. It's a solid at room temperature, so inhalation exposure risk is greatly reduced. Ingestion, skin or eye contact can still cause negative health effects. Inhalation of vapor is still a risk, even for solids, so handlers should always use masks, as well as gloves and goggles. Iodine is not flammable, but heating creates toxic fumes.
Astatine
Astatine is also a solid at standard conditions, so inhalation risk is reduced but not eliminated in comparison with the gaseous and liquid halogens. Astatine is the only halogen that's radioactive, which presents unique safety concerns for storage and transportation. These safety concerns are mostly theoretical. The substance is considered the rarest substance on Earth. There is only a tiny amount in existence, unlike the other four halogens that are produced in much greater quantities.