Caesium is a chemical element with symbol Cs and atomic number 55. [1] It is silvery gold, soft, and ductile. It is the most electropositive and most alkaline element. Caesium, gallium, and mercury are the only three metals that are liquid at or around room temperature. Caesium reacts explosively with cold water and reacts with ice at temperatures above -116°C. Caesium hydroxide is a strong base and attacks glass. Caesium reacts with the halogens to form a fluoride, chloride, bromide, and iodide. Caesium metal oxidises rapidly when exposed to the air and can form the dangerous superoxide on its surface. [2] Caesium is a naturally-occurring element found in rocks, soil, and dust at low concentrations. Natural caesium is present in the environment in only one stable form, as the isotope 133Cs. [3]

Uses [2]

Caesium is used in industry as a catalyst promoter, boosting the performance of other metal oxides in the capacity and for the hydrogenation of organic compounds. Caesium nitrate is used to make optical glasses. Caesium is sometimes used to remove traces of oxygen from the vacuum tubes and from light bulbs. Caesium salts are used to strengthen various types of glass. The chloride is used in photoelectric cells, in optical instruments, and in increasing the sensitivity of electron tubes. Caesium is used in atomic clocks and more recently in ion propulsion systems.

Sources & Routes of Exposure [3,4]

Sources of Exposure

You can be exposed to low levels of stable or radioactive caesium by breathing air, drinking water, or eating food containing caesium.

Food and drinking water are the largest sources of exposure to caesium.

You can be exposed to radioactive caesium if you eat food that was grown in contaminated soil, or if you come near a source of radioactive caesium.

Working in industries that process or use natural caesium or caesium compounds.

Living near uncontrolled radioactive waste sites containing caesium.

Routes of Exposure

Stable and radioactive caesium can enter your body from the food you eat or the water you drink, from the air you breathe, or from contact with your skin. When you eat, drink, breathe, or touch things containing caesium compounds that can easily be dissolved in water, caesium enters your blood and is carried to all parts of your body. Caesium is like potassium; it enters cells and helps to maintain a balance of electrical charges between the inside and the outside of cells so that cells can perform tasks that depend on those electrical charges. Cells like muscle cells and nerve cells require changing electrical charges in order to function properly and allow you to think and move. Once caesium enters your body, your kidneys begin to remove it from the blood; some caesium is quickly released from your body in the urine. A small portion is also released in the faeces. Some of the caesium that your body absorbs can remain in your body for weeks or months but is slowly eliminated from your body through the urine and faeces.

Health Effects [4]

Effects of Caesium

It is highly unlikely that you would be exposed to high enough amounts of stable caesium to cause harmful health effects. Laboratory animals given very large amounts of caesium compounds showed changes in behaviour, such as increased or decreased activity. Exposure to large amounts of radioactive caesium can damage cells in your body from the radiation. You might also experience acute radiation syndrome, which includes nausea, vomiting, diarrhoea, bleeding, coma, and even death in cases of very high exposures.


There are no studies regarding non-radioactive caesium and cancer. There are no human studies that specifically associate exposure to radioactive caesium with increased cancer risk. Because radioactive caesium emits ionising radiation, carcinogenic effects similar to those observed in Japanese survivors of the atomic bombing incidents might be expected among individuals acutely exposed to very high levels of radiation from a radioactive caesium source. Rats exposed to high doses of radiation from 137Cs had increased risk of mammary tumours. Older rats seemed more resistant than younger ones.

Safety [5]

First Aid Measures

Eyes: Flush eyes with plenty of water for at least 15 minutes, occasionally lifting the upper and lower eyelids. Get medical aid immediately.

Skin: Get medical aid immediately. Flush skin with plenty of water for at least 15 minutes while removing contaminated clothing and shoes.

Ingestion: If victim is conscious and alert, give 2-4 cupfuls of milk or water. Get medical aid immediately.

Inhalation: Get medical aid immediately. Remove from exposure and move to fresh air immediately. If not breathing, give artificial respiration. If breathing is difficult, give oxygen.

Notes to Physician: Treat symptomatically.

Fires & Explosion Information

As in any fire, wear a self-contained breathing apparatus in pressure-demand, MSHA/NIOSH (approved or equivalent), and full protective gear.

Caesium can burn in a fire, releasing toxic vapours.

Caesium will react with water and may release a flammable and/or toxic gas.

May ignite or explode on contact with steam or moist air.

Do NOT use water directly on fire.

Exposure Controls & Personal Protection

Engineering Controls

Use explosion-proof ventilation equipment.

Use process enclosure, local exhaust ventilation, or other engineering controls to control airborne levels.

Personal Protective Equipment

The following personal protective equipment is recommended when handling caesium:

Eyes: Wear appropriate protective eyeglasses or chemical safety goggles as described by OSHA’s eye and face protection regulations in 29 CFR 1910.133 or European Standard EN166.

Skin: Wear appropriate protective gloves and clothing to prevent skin exposure.

Clothing: Wear appropriate protective clothing to minimize contact with skin.

Respirators: Follow the OSHA respirator regulations found in 29 CFR 1910.134 or European Standard EN 149. Always use a NIOSH or European Standard EN 149 approved respirator when necessary.


United States [3]

There are few guidelines for compounds of stable caesium.

NIOSH: Based on eye irritation, the National Institute for Occupational Safety & Health has established a recommended exposure limit (REL) for caesium hydroxide of 2 mg/m3 as a time-weighted average (TWA) for up to a 10-hour workday and a 40-hour workweek.

ACGIH: The American Conference of Governmental Industrial Hygienists has assigned caesium hydroxide a threshold limit value (TLV) of 2 mg/m3 as a TWA for a normal 8-hour workday and a 40-hour workweek, based on respiratory and eye irritation.

NRC: The Nuclear Regulatory Commission has established guidelines for radioactive caesium that include occupational inhalation exposure Derived Air Concentrations (DACs) of 0.00000004 ìCi/mL (4×10-8 ìCi/mL) for 134Cs and 0.00000006 ìCi/mL (6×10-8 ìCi/mL) for 137Cs. Annual Limits on Intake (ALIs) for on-the-job exposure are 100 ìCi (1×102 ìCi) for 134Cs and 200 ìCi (2×102 ìCi) for 137Cs.

Australia [6]

Safe Work Australia: Safe Work Australia has established an 8-hour time-weighted average concentration for caesium of 2mg/m3.