Safety Tools

Ionization Smoke Detectors

Have you wondered how ionization detectors work? This focuses on ionization smoke detectors. With the aid of this guide, you should have an idea of how ionization smoke detectors work. The importance of knowing how they work is that you would be able to determine if they are a good fit for you and your household. Therefore, you should endeavor to read through so you can gather all the information you need about ionization smoke detectors.

What is a smoke detector?

Smoke detectors are common household items that keep you and your family safe by alerting you to smoke in your home. All smoke detectors consist of two basic parts: a sensor to sense the smoke and a very loud electronic horn to wake people up.

Smoke detectors can run off of a 9-volt battery or 120-volt house current. While it can be easy to forget to check and test smoke alarms regularly, it is important to ensure that they are not only properly maintained but also correctly installed.

How do Smoke Detectors Work?

Since smoke rises, smoke detectors are always placed on the roof for ideal detection. There are two common types of smoke detectors to choose from photoelectric and ionization. Photoelectric sensors use a beam of light within the detector to sense any smoke in the air.

The beam will shoot in a straight line when the air is clear. In the presence of smoke, the light will scatter, this is when the alarm will go off. Ionization detectors, on the other hand, use a chamber with a small electrical current. When smoke enters the chamber and the level of ions in the air changes, the electrical current drops and the alarm will sound.

Each function is beneficial in different instances and because of this, one is not necessarily better than the other. It is, however, recommended to have one of each of these functions on each floor of your home and in high-risk areas, such as the kitchen or near fireplaces and heating appliances.

A typical household smoke detector is disc-shaped and powered by a disposable battery. There are two types of smoke detectors – photoelectric and ionizing.

Generally, home smoke detectors work in one of two ways:

Ionization detectors respond best to fast, raging fires and flames. They use a small amount of radioactive material passing between two electrically charged plates to create an ionization chamber. This ionizes the air and creates a current that flows between the plates. Smoke particles disrupt the flow of ions, reducing the current and triggering the alarm.

Photoelectric detectors respond best to smoky fires that start with a long period of smoldering. They rely on a photoelectric sensor and a light source to detect smoke. When smoke seeps into a compartment in the detector, it covers the light beam and reflects light onto the light sensor, causing the alarm to sound.


Ionization smoke detectors use a small amount of radioactive material, americium-241, which ionize smoke particles in the air. This type of smoke detector is more common because it is inexpensive and better at detecting the smaller amounts of smoke produced by flaming fires.

Inside an ionization detector is a small amount (perhaps 1/5000th of a gram) of americium-241. The radioactive element americium has a half-life of 432 years and is a good source of alpha particles.

The functionality principle of the radioactive isotope americium-241 involves the emission of alpha particles, which ionize air particles in the sensing chamber so that they conduct electricity.

During the emission of the alpha particles into the air, a chemical reaction occurs where air molecules in the collecting chamber are ionized and, as a result, carry an electrical charge. Smoke particles surrounding the ionizing smoke detector enter the collecting chamber and attach to the ions, making it more difficult for the ions to deliver an electrical charge.

A decrease in the electrical flow is detected by a circuit that then triggers an alarm system. This type of smoke detector is ideal for detecting invisible particles produced by fires, and so will work much faster and be more sensitive compared to a photoelectric detector.

Alpha particles are used for the ionizing smoke detector as they have high ionizing potential. Although there are concerns that this type of smoke detector emits radiation, the alpha particles used have low penetrative potential and so will not pass through plastic material.

The standard amount of americium-241 used in the ionizing smoke detector is not enough to raise regulatory concerns – a standard amount of radioactive americium-241 is made up of 37KBq or 1ìCi, which is enough to detect smoke particles without contaminating the outer surface of the detector.

Another way to talk about the amount of americium in the detector is to say that a typical detector contains 0.9 microcuries of americium-241. A curie is a unit of measure for nuclear material. If you are holding a curie of something in your hand, you are holding an amount of material that undergoes 37,000,000,000 nuclear transformations per second.

Generally, that means that 37 billion atoms in the sample are decaying and emitting a particle of nuclear radiation (such as an alpha particle) per second. One gram of the element radium generates approximately 1 curie of activity (Marie Curie, the woman after whom the curie is named, did much of her research using radium).

Before being sold on the market, ionizing smoke detectors go through a radiation safety analysis to meet safety requirements. Compared to photoelectric smoke detectors, the ionizing smoke sensor is less expensive and more commonly used.

It is important to note that The ionization smoke detector has two ionization chambers, one open to the air, and a reference chamber that does not allow the entry of particles. The radioactive source emits alpha particles into both chambers, which ionizes some air molecules.

There is a potential difference (voltage) between pairs of electrodes in the chambers; the electrical charge on the ions allows an electric current to flow. The currents in both chambers should be the same as they are equally affected by air pressure, temperature, and the aging of the source. If any smoke particles enter the open chamber, some of the ions will attach to the particles and not be available to carry the current in that chamber.

An electronic circuit detects that a current difference has developed between the open and sealed chambers, and sounds the alarm. The circuitry also monitors the battery used to supply or back up power and sounds an intermittent warning when it nears exhaustion.

A user-operated test button simulates an imbalance between the ionization chambers and sounds the alarm if and only if the power supply, electronics, and alarm device are functional. The current drawn by an ionization smoke detector is low enough for a small battery used as a sole or backup power supply to be able to provide power for months or years without the need for external wiring.

Ionization smoke detectors are usually cheaper to manufacture than optical detectors. They may be more prone to false alarms triggered by non-hazardous events than photoelectric detectors and are much slower to respond to typical house fires.


  • Cheaper than photoelectric smoke sensors
  • Ionizing smoke alarms outperform photoelectric smoke alarms when detecting fast flaming fires with little visible smoke


  • Very sensitive, which can lead to false alarms as a product of cooking
  • Not as responsive to smoldering fires – they are minutes slower than photoelectric sensors in detecting smoke particles from smoldering fires
  • Some European countries, including France, and some US states and municipalities have banned the use of domestic ionic smoke alarms because of concerns that they are not reliable enough as compared to other technologies. Where an ionizing smoke detector has been the only detector, fires in the early stages have not always been effectively detected.
  • The use of radioactive material is a concern
  • The biggest disadvantage is that it contains a very small amount of radioactive material. These devices are probably not a problem unless they burn, in which case the Americium would be released in particulate form – an inhalation hazard.
  • Another big disadvantage is that it is a bit too sensitive. It can be vulnerable to nuisance alarms if placed too close to cooking. It goes off many times especially if it is kept near a kitchen. This can be very annoying especially if it happens repeatedly.

What Type of Fires Does an Ionization Alarm Detect?

Generally, ionization alarms are more responsive to flaming fires. The term “flaming” fires refers to fires resulting from flammable liquids, wood, or paper starting on fire. This type of fire produces a lot of flames with a limited amount of smoke.

Most house fires are categorized as fast-flaming fires, which is why ionization alarms are popular in homes. To be safe, we recommend having both types of home fire detectors installed, or installing combination alarms that detect multiple types of fires.

What You Can Do

  • Use a smoke detector in your home. It can save your life.
  • Never tamper with an ionization smoke detector. Never attempt to tamper with or remove the americium.
  • Replace the batteries. Replace the batteries in every smoke detector in your home twice a year. Most detectors are certified for a useful life of ten years. Check the expiration date on your smoke detector when you replace the batteries.
  • Throw away outdated ionization smoke detectors. Your community may have a separate recycling program for them.


Having gone through this review, you know what you stand to gain when you get an ionization smoke detector, and also what loss you’ll have. Nevertheless, it is always important to own a smoke detector in your home.