Smoke alarms – Photoelectric or Ionization (Part 2)

By Ed Comeau, Publisher, Campus Firewatch

Copyright 2008©

This article is a continuation of the discussion started in the February issue of Campus Firewatch.

The debate about what type of smoke alarm is the best or appropriate one to use rages on with no immediate resolution in site, unfortunately. Given that over 80 percent of the campus-related fatal fires occur in the off-campus environment, this is a vitally important issue to the campus fire safety professional and fire chiefs that have students living in their communities.

What the debate is about

There are two types of smoke alarms that use different technologies – photoelectric and ionization. Photoelectric smoke alarms work by focusing a beam of light into a sensing chamber and as smoke enters the chamber the light is reflected off of the particles onto a sensor. When a certain level of obscuration is reached, and therefore a certain level of light is reflected onto the sensor, the smoke alarm sounds an alarm. Ionization works by ionizing the air between two sensor plates. As the particles enter this space, the current flow is reduced in the combustion chamber. When a threshold level is reached, the alarm is then activated.

What has been known for some time is that photoelectric smoke alarms work best at detecting smoldering fires because the particle size tends to be larger and ionization are more effective at sensing open-flaming fires because the particle size is smaller.

Ionization smoke alarms are the ones that are in widespread use across the nation and there have been a number of reports in the media regarding the effectiveness of these devices in detecting the typical fire. In 2004, NIST did a series of tests in a one-story manufactured home and a two-story residence. These tests involved an array of smoke alarms in various locations throughout the buildings and fires that simulated an open-flaming fire, such as one started by a candle, and a smoldering fire which would be similar to a cigarette in a seat cushion.

The arrays of smoke alarms were a mixture of single-sensor photoelectric and single-sensor ionization. Dual-sensor smoke alarms were used as well that included both types of technology in a single unit. However, as it turned out, these were not co-located with the single-sensor units, leading to erroneous reporting of the activation times.

The NIST study has been given a lot of scrutiny. Since it was first published in 2004, it has since been reissued twice and had a number of revisions done to the reporting and the data.

In an interview, Dr. William Grosshandler, deputy director of the Building and Fire Research Laboratory at NIST, reported that the major objectives purpose of the test was “see the level of protection provided by smoke alarm siting…to compare the performance of a single smoke alarm with a detector on each level and in each bedroom …and to see if increasing the number of smoke alarms would increase the level of life safety. That was the major objective.”

This is in contrast to the NIST report which states in its Executive Summary, “In the past few years questions were being raised about the efficacy of some smoke alarm technologies, whether the numbers and locations in homes still represented the optimum configuration, and if multi-sensor designs (as are becoming popular for commercial fire alarm systems) might perform better or produce fewer nuisance alarms. Some simply thought that it was time to reexamine the technology in light of its importance to public safety.”

An article in the February issue of Campus Firewatch covered the information in this report and a special reprint of this article has been made available on the Campus Firewatch website at www.campus-firewatch.com.

NFPA Task Group Report

The National Fire Protection Association formed a task group to look into this issue and recently released the report Minimum Performance Requirements for Smoke Alarm Detection Technology. This report covers the performance of smoke alarms as outlined in the NIST report and discusses the tenability issues associated with attempting to determine what is the minimum required alarm time. In reviewing this report, there were several questions raised regarding the information in it and the methodology.

“This is a technical document for the committee (that oversees the standard for smoke alarms),” said Chris Dubay, vice president of Codes and Standards for the NFPA. “It was not intended to be a public relations piece, it is a technical document.” He went on to explain that this is just one document that will be used in the committee’s deliberation process in reviewing the standard (NFPA 72). “The Task Force was tasked with getting data and understanding what the current issue is between photo and ionization and what is the research that is available for the full committee to make decision.”

The Task Group Report reinforces that photoelectric smoke alarms are better suited for detecting smoldering fires while ionization smoke alarms have a faster response time reacting to open-flaming fires, which is well-established. One of the points of discussion in this report which is at the basis of much of the controversy, is how quickly untenable conditions are arrived at in fires. Given that today’s fires burn much hotter and faster than in the past due to the proliferation of synthetic materials in furnishings, one would think that it would be critically important to detect a fire in its earliest stage. However, it would appear that the Task Group Report takes a different view on this topic.

“The current assumptions of Chapter 11 of NFPA 72, ‘Single-and-Multiple-Station Alarms and Household Fire Alarm Systems’, are key to judging the performance of smoke detectors to serve their purpose, which is stated as follows:

‘Fire warning equipment for residential occupancies shall provide a reliable means to notify the occupants of the presence of a threatening fire (emphasis added) and the need to escape to a place of safety before such escape might be impeded by untenable conditions in the normal path of egress.’” (Note that the emphasis added was done in the original report and this is a verbatim reproduction.)

When asked what was meant by the term “threatening fire,” Dubay replied, “There are two reasons. One is a fire that looks bad, may not be. The question is tenability. What do we need to alert people to, to get them out?” He went on to explain that “There are a lot of ‘normal’ fires such as cooking, candles, fireplace that we don’t need to alert to. That is why we highlighted that term.”

Included in the report were several dissenting opinions offered by members of the Task Group who did not agree with the conclusions of the report. One of them, by Bob Bourke with the Lynn (MA) Fire Department, discussed this very issue.

“Perhaps the assumptions in NFPA 72 should be revised by deleting the word ‘threatening’ from the performance objective. What type of home fire that is not intentionally ignited is not ultimately threatening? Does NFPA 72 want to take a position that an ignited fire that does not develop into a life threatening fire should go unnoticed until the charred debris is discovered the next day?”

Adding to this were comments by Dr. Don Russell from Texas A&M.

“When I read the analysis of the NIST data it appears to state that it is acceptable to stay in a residence that is on fire for a longer time than necessary because we are establishing the criteria of success as adequate time to escape before conditions become untenable. I disagree with this philosophy when establishing performance objectives for fire detection.

“Our criteria should be that we provide the fastest possible warning of the presence of a fire in a residence within the constraints of the technologies available to us for application. All residents should be warned of a fire as fast as possible. The combination of types of detectors, location of detectors, sensitivity of detectors, etc. should be optimized for the sole purpose of the fastest possible warning. Our draft report understates the impact of visibility and smoke obscuration on tenability and understates the combined impact of smoke and toxic gases on the behavior of residents.”

The Task Group Report also discussed some of the assumptions regarding NFPA 72’s requirement for the use and placement of smoke alarms. Two studies were mentioned several times in the report and apparently given some weight.

“The findings of two studies may be of equal or perhaps more importance in understanding reasons for perceived inadequate smoke alarm performance. The National Smoke Detector Project found that 26% of the households surveyed had fewer than one alarm per floor. A 2000 study of 691 homes in rural Iowa found that smoke alarms were not installed according to NFPA requirements in 57% of the homes with smoke alarms.”

While the cited reference in the Task Group Report was dated 2007, when Campus Firewatch researched the original source report “The National Smoke Detector Project,” it was found that this was a study that was done in 1992 and 1993 and published in 1995. It looked at 263 fires in 15 cities where 14 fire deaths had occurred. It would have to be assumed that smoke alarm usage has changed (for better or worse) in fifteen years and it may be erroneous to base the installation assumptions on such a dated study. In addition, the size of the population surveyed in both the National Smoke Detector Project and the one done of the 691 rural homes in Iowa is a relatively small one and not necessarily representative of the population-at-large.

The Report also discusses the assumptions used in NFPA 72 installation criteria.

• Occupants are not intimate with the ignition and are capable of self-rescue.
• Occupants have an escape plan and ability to execute the plan.
• An escape route is available to occupants, and the route is unobstructed prior to the event of the fire.
• Smoke alarms are installed and maintained operable in accordance with Chapter 11 of NFPA 72.

The report clarifies the final item as “This means that interconnected smoke alarms are installed on every level of the home and in each bedroom.”

In the dissenting opinions, Bourke and Russell state that it is unrealistic to expect that today’s homes and occupants meet the criteria outlined in NFPA 72.

According to Russell, “While interconnection of smoke detectors is an appropriate goal and reasonable objective for the future, this will not be the majority practice for a long time. Interconnection of smoke alarms in residences should never be assumed in any way in the evaluation of performance data of individual smoke detectors. Given the state of application of residential smoke detectors in the United States, we should make every attempt to optimize the performance of each individual stand-alone detector as to both detection sensitivity and reliability. I am not sure these are the assumptions we have used in the draft report.”

Bourke adds, “The assumption that the occupants of the home have an escape plan is not at all realistic. In reality the only occupancies that have a true escape plan and in fact practice it are primarily health care and educational use groups.”

Dual Sensor Smoke Alarms

In addition to single-sensor smoke alarms, manufacturers offer dual-sensor smoke alarms that have both photoelectric and ionization technologies included in a single unit. A number of fire professionals had been looking at this as a possible solution to the “which type of technology” dilemma, but it appears that this may not be the answer at this time.

According to industry representatives, it is possible to adjust the sensitivity of each of the sensors in a dual-sensor smoke alarm to different levels than those of a single-sensor smoke alarm and still stay within the Underwriters Laboratories listing criteria. This means that the ability of a dual-sensor smoke alarm may not necessarily mirror that of a single-sensor smoke alarm.

When NIST did their series of tests that were reported in 2004, the results for dual-sensor smoke alarms were included in the results alongside the response times of the single-sensor smoke alarms. When a review of this data was done by Campus Firewatch, it was observed that the response times of the dual-sensor smoke alarms were, in some cases, significantly worse than those of the comparable single-sensor alarms.

When NIST was questioned on this disparity, they reported that the dual-sensor smoke alarms had not been co-located with the single-sensor smoke alarms which was the reason for the different response times. As a result, when the report was reissued for a second time in February 2008, the data reporting the performance of the dual-sensor smoke alarms was removed from the tables when they had not been co-located. Unfortunately, the number of co-located dual-sensor and single-sensor smoke alarms is too small to be able to draw an assumption on their performance.

Tenability

Much of the critical information in both the NIST and NFPA report relates to the amount of time for untenable conditions to occur. This is a complex issue that will be covered in a future issue of Campus Firewatch.

Additional Information

Links to both the NIST and NFPA reports, as well as podcasts on this topic, are available at Campus Firewatch at www.campus-firewatch.com. The International Association of Fire Chiefs, Underwriters Laboratories, NFPA and others are continuing to evaluate this situation.

Ed Comeau has been the publisher of Campus Firewatch since its founding in 2000. He is also a board member of the International Association of Fire Chiefs Fire and Life Safety Section, a board member of the NFPA Lodging Section and a contributing author to the NFPA Fire Protection Handbook on campus fire safety. He was the founding director of the non-profit Center for Campus Fire Safety and the ongoing Campus Fire Forums. He was the chief fire investigator for the National Fire Protection Association and a fire protection engineer with the Phoenix Fire Department’s Special Operations and Training Division. He was a member of the Amherst Fire Department while obtaining his degree in Civil Engineering at the University of Massachusetts.

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