Table 2 Research findings from the tobacco industry (www.coresta.org)

Factors affecting operation or accuracy of ASTM E2187 test outcomes

A smouldering cigarette may burn close to the cigarette tipping paper (the paper wrapper connecting the filter with the tobacco rod). When this occurs, the operator has to make a subjective judgment on whether the test piece has “passed” or “failed” the test, because the paper char line may display a gradient of charring across ≥1 mm.

The cigarette paper that wraps the tobacco rod has a longitudinal seam. Its orientation relative to the contact point with the substrate may affect test results because the seamed region has different gas diffusion and/or heat conduction properties. The standard test methodaccommodates this by specifying that all test cigarettes must have their seams facing upward during the test.

Test results depend on the “side” (i.e. surface roughness) of the substrate paper (Whatman No. 2 filter paper). Simply specifying “Whatman No. 2 filter paper” is also not sufficient to guarantee test performance because systematic differences in the test results were observed when Whatman relocated its production site.

Prototype automatic or semi-automatic test machines have been designed to improve the efficiency and consistency of the test.

Band parameters and effects

Band diffusivity values have become the most important band specification parameter. Commercial instruments have been designed to allow both the air permeability and diffusivity of the band to be determined with good accuracy. The band width is usually 5–7 mm, spaced between 16 and 20 mm apart, and invisible to the naked eye. These values are derived through systematic experiments and/or computer modelling. The position of the bands are randomly distributed along the length of the tobacco rod, but usually two complete bands are present for a 57-mm tobacco rod (the most common format of cigarettes) (Codwise et al. 2006; Eitzinger 2006a, 2006b; Eitzinger 2008).

Thermophysical studies on a steady-state smouldering cigarette in a 10-layer filter paper test noted that there was a significantly modified temperature distribution inside the burning coal of the banded cigarette. The top three layers suffered severe thermal decomposition around the contact region with the burning coal, and overall the substrate acted essentially as a heat sink with no intrinsic sustained smouldering or flame combustion.

Correlations with other cigarette measurements

The free-air self-extinguishment (FASE) rate is the percentage of cigarettes that go out during free smouldering. Free smouldering is defined as a smouldering cigarette that burns without intermittent puffs applied to it. FASE follows a relationship opposite to that of band permeability to control the RIP “pass rate” based on 75 % of test pieces extinguish before reaching their full length (Fig. 3). Studies have shown that a band permeability of 10–15 CORESTA units (cm³ min⁻¹ kPa⁻¹) statistically satisfies both RIP performance and an acceptable FASE rate. An air permeability window of 5 CORESTA units has to be carefully controlled to ensure that different tobacco blends are accommodated to meet the RIP target. This value also needs to be coordinated with properties of the base paper to maintain set mainstream smoke yields.

A correlation exists between percentage full-length burn measurement obtained during the standard RIP test and the mean burn length (the average tobacco rod length burnt during the RIP test)—a relationship that helps to evaluate the effect of cigarette design parameters for those cigarettes that consistently achieve a “pass rate” of 100 %.

Other studies

In addition to the ignition propensity regulation, RIP cigarettes have to meet existing regulations on smoke yields. For example, all cigarettes sold in the European Union have to meet the limits of 10 mg of tar, 1 mg of nicotine and 10 mg of carbon monoxide in mainstream smoke, as measured by International Organization for Standardization (ISO) standard machine-smoking parameters (ISO3308 2012). To mitigate the impact of adding bands on the cigarette paper, which restricts oxygen availability and increases the amount of organic material to burn during a puff, cigarette manufacturers usually make systematic adjustments to the base paper properties and/or tobacco blend compositions.

Different statistical approaches and uncertainties have been associated with some of the short-term (e.g., 1- or 2-year) fire statistics reported since the introduction of RIP regulation.