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If organic material is extremely hot, smouldering, or flaming it can be a significant ignition source. One type of organic ember is produced by accumulated debris on vehicles. For example, 4WD vehicles or machinery can trap mud and grass on hot parts of the exhaust or braking systems. It is possible for the hot organic material to fall off and ignite grass fuels.

Burning cigarettes are another common type of organic ember, and can reach temperatures between 600 and 700ºC, depending on the brand (Redsicker & O'Connor, 1997; Steensland, 2005). This section reviews ignitions from accumulated debris on hot vehicle parts, and from cigarettes.

Baxter (2004) carried out experiments using two ATVs that were sunk into muskeg

vegetation of mucky consistency to accumulate organic material around the operating system.

They were then driven for eight minutes in an ambient temperature of 15ºC. By this time, the accumulated vegetation had dried and begun smouldering at several points along the exhaust system. Within 15 minutes the smouldering vegetation began dropping onto the ground.

Baxter (2004) also conducted a survey of ATV riders, and 52% of respondents reported that they had smelt burning from their own or another person‘s ATV.

The ignition risk from cigarettes is commonly misinterpreted. The NRFA recorded that only 1% of New Zealand‘s wildfires (1991-2007) were caused by cigarettes, and this only contributed to 0.2% of New Zealand‘s grass fires (Doherty et al., 2008). Galtié (2006) reported that ignitions from cigarettes are much easier to achieve in controlled laboratory

conditions, with low likelihood of occurring in a natural environment. Redsicker and

O‘Connor (1997) suggested that fires caused by cigarettes are subject to three elements, each with decreasing likelihood: 1) the burning cigarette and fuel must be in contact; 2) the fuel source must be in a state favourable for ignition; and 3) the cigarette‘s position must be inclined to ignite the fuel. Furthermore, ignition probability depends on the smoker‘s character. The smoker must discard the cigarette carelessly for it to be a potential ignition source.

The NRFA (2007) provides fire investigators with an indication of conditions which are conducive to cigarette fires (Table 2.3). For ignition to occur, ambient temperature must be high, RH must be less than 22%, and wind speed must not be too fast or too slow. An ideal wind speed of 1.3 m/s has been reported (Countryman, 1983; Steensland, 2005). Furthermore, dead fuel MC must be less than 14%. Research indicated that ignitions are more likely to occur if fuel arrangement is horizontal, fuel moisture is low, fuel is cured, fuel density is high, and fuel particles are small (Sale & Hoffheins, 1928; Countryman, 1983; Redsicker &

O'Connor, 1997; Galtié, 2006; National Rural Fire Authority, 2007). Once these criteria are met, the cigarette must land in the fuel at the correct orientation. Ford (1995) maintains that at least a third of the cigarette must contact the fuel for ignition to occur. Moreover, when cigarettes burn, ash surrounds the ember where glowing combustion is taking place. This ash acts as a buffer between the ember and the fuel, further decreasing ignition probability (Steensland, 2005).

Table 2.3 Indicators for wildfires caused by smoking cigarettes (from National Rural Fire Authority, 2007).

Cigarette Caused Fires

PROBABILITY OF IGNITION BASED ON RELATIVE HUMIDITY

Fuel conditions critically dry:

RH less than 22% and fine dead fuel moisture less than 14%.

Start Start Start No Likely Possible Unlikely Start 0% RH 10% RH 18% RH 22 % RH

Sale and Hoffheins (1928) determined the probability of dry grass ignition from various brands of cigarettes and cigars (Table 2.4). Cigarettes and cigars were lit and placed into a wire cage full of dry grass at MC < 12%. Without wind, no ignitions occurred, but as wind speed increased so did ignition probability. At 2.8 m/s, the probability of ignition was 50% for

a grass bed of 0.043 g/cm³ density. This increased to 80% for a grass bed of 0.099 g/cm³ density.

Ignition probability of cigars was about half that of cigarettes, and did not increase any further when wind speed was increased. This was probably because cigarettes contain an additive that aids in glowing combustion, whilst cigars do not (Redsicker & O'Connor, 1997). Ignition times varied between five and 11 minutes, where higher wind velocities were conducive to faster ignition times (Sale & Hoffheins, 1928).

Countryman (1983) investigated the ignitibility of dead cheat grass (Bromus tectorum L.) from cigarettes (Table 2.4). Tests were conducted in a controlled chamber at 27ºC, with a 1.3 m/s wind speed. RH was varied to condition grass samples to different MC values. Samples included three size classes based on grass length, fine (< 0.3 cm), medium (0.3 to 0.5 cm), and coarse (1.9 to 3.8 cm). Cigarettes were placed onto the samples at three orientations, towards the wind, at right angles to the wind, and away from the wind. No ignition occurred for coarse fuel. Some ignitions and many marginal ignitions occurred for medium fuel, but no clear trends were observed. The ignition threshold for fine fuel suggested that ignition occurs below 14% MC. Deeper fuel beds increased the probability of ignition slightly. The study reported that when facing towards the wind, the probability of ignition from a burning cigarette was higher compared with other orientations.

The results from this study indicated that the likelihood of cigarettes igniting long grass in its natural environment is low (Countryman, 1983).

Table 2.4 Details of experiments investigating fire ignition from cigarettes and cigars.

Experim ent

Description Ignition Source Sam ple Type Ignition Threshold or

Probability Reference

For cigarettes w ith grass at 0.043 g/cm³ density:

0% for 0.7 m/s 5% for 1.4 m/s 30% for 2.0 m/s 50% for 2.8 m/s If w ind increased above 2.8 m/s ignition probability did not increase, but if grass density increased to 0.099 g/cm³, ignition probability became 80%,

Ignition probability w as about half for cigar tests

Conditions w ere 27ºC and 1.3 m/s w ind speed. Fine fuel: < 13.5% MC, Medium fuel: no clear trend, but can assume w ith confidence < 14% MC, Coarse fuel: No ignition at any MC

Countryman 1980

Babrauskas (2003) noted that when dried cow dung is pulverized by cow hooves, it can be ignited by cigarette butts. Once ignited, the fire may spread to surrounding grassland.