Current Concepts towards the Health Hazards of BBQ Smoke and Newer Possibilities for Risk Mitigation: A Pilot Study

The toxic and unhealthy effects of Barbecue (BBQ) smoke is well known. Incomplete combustion of energy sources such as charcoal and wood chips give rise to Volatile Organic Compounds (VOCs) such as toluene and formaldehyde. They also give rise to particulate matter of small caliber e.g. PM25 that readily enter the pulmonary system. The net effect is a substantial risk of pulmonary diseases and possible carcinogenic effects. However, little progress has been made in terms of finding a way to cleanse the smoke of these potential hazards while at the same time preserving the ingredients of BBQ that prompt its use in the first place. Syringol and Guaiacol are two VOCs that provide the desirable smell and taste for the BBQ user. Here we review the progress to date of the steps that have been taken to solve this health concern and then propose a novel plan to execute differential scrubbing of BBQ smoke. It is suggested that activated carbon will preferentially adsorb more of the main body of VOCs than Syringol and Guaiacol simply because the latter two compounds are phenols which happen to be polar molecules. Activated carbon used for scrubbers is non-polar and tends to adsorb non-polar VOCs rather than polar VOCs.

A pilot experiment was performed to exhibit the predicted differential adsorption. Smoke generated from an ordinary BBQ grill was allowed to be scrubbed by a high concentration that removed all VOCs including Syringol. The Syringol was monitored indirectly by an olfaction test. A 50% reduction in carbon concentration resulted in a dramatic differential reduction of the VOCs with a much greater adsorption of overall VOCs in comparison to Syringol. The results are encouraging in that this differential adsorption process does provide a potential to rid much of the toxic VOCs from common BBQ practice. Future studies are suggested including the design of a carbon scrubber system for the common BBQ grill.

The barbecue (BBQ) passion

It is plainly evident that our country has a passion for grilling. For many people, the smell of a charcoal grill is strong enough to conjure up a video of happy summer memories made with the people they adore. In some parts of the country, grilling competitions bare a resemblance to sporting events. Unfortunately, most people probably do not consider the potential dangers they are exposing themselves to when inhaling substantial amounts of barbecue smoke. And most probably do not consider the potential toxic effects that smoke has on their cooked food despite the fact that it provides an undeniable savory and welcome taste to that food that is not attainable by any other method. The American citizen pursues his/herB (BBQ) experience despite knowledge that many warnings have been given by governmental entities and medical organizations that the smoke emanating from a BBQ is not healthy. Specifically, there is a substantial risk of pulmonary diseases and possible carcinogenic effects from BBQ smoke. The toxic fumes created during a barbecue do not only affect those in attendance. One 2012 study for the State of California Air Resources Board found that summertime grilling across different homes in an area can cause emissions to accumulate and impact the health of everyone in the community [1].

For quite some time, charcoal grilling, whether in the form of domestic barbecues or at charcoal restaurants, has become popular due to the unique smoky flavor and smell it provides food [2-5]. Despite the availability of various alternative grilling fuels such as gas, and electricity, charcoal-based products continue to dominate as the most widely used option for barbecue activities [2,6]. As noted by Seboka (2009) [7] charcoal derives from the pyrolysis process of various types of biomasses, primarily wood, conducted at relatively low temperatures and reduced oxygen concentration. It offers several advantages compared to wood, including higher calorific value, faster ignition times, and ease of transport and storage [8]. Unfortunately, charcoal is a major source of the health risk in question as are wood products such as wood chips.

Health burden as a result of bbq smoke

The toxic and unhealthy effects of Barbecue (BBQ) smoke is well known to the medical profession and chemical engineers. Incomplete combustion of energy sources such as charcoal and wood chips give rise to Volatile Organic Compounds (VOCs) such as toluene, formaldehyde, acetaldehyde, and benzene. They also give rise to particulate matter of small caliber, e.g. PM_2.5 that readily enters the pulmonary system like dust particles. These VOCs are known to have an adverse impact on human health, including lung irritation, bronchitis, asthma, and cancer. Consequently, preventive measures, such as filters or scrubbers have been recommended to reduce the inhalation of VOCs Zhou X, et al. [9]. Table 1 shows that ambient VOC concentrations > 1.0 mg/m3 may have toxic effects.

Importantly, the health risks are potentially much more severe than those simple numbers. The use of charcoal to grill foods causes the release of high concentrations of pollutants such as Polycyclic Aromatic Hydrocarbons (PAHs) not just Particulate Matter (PM) and VOCs. Besides contributing to the deterioration of air quality, emissions can seriously affect human health in other ways. Inhalation of charcoal and wood combustion fumes can lead to short term effects determining cardiovascular issues, altering blood pressure, and promoting heart rhythm disturbances [10]. Prolonged exposure to biomass combustion fumes can cause the insurgence of respiratory inflammation, reduced lung function, and the insurgence of Chronic Obstructive Pulmonary Disease (COPD), with symptoms of cough, wheezing, mucous hyperproduction, dyspnea, and chronic bronchitis [10,11]. Lenssen ES, et al. [12] provided a detailed assessment by investigating short-term changes in respiratory health in healthy adults who were exposed to barbecue fumes. Sixteen healthy, adult volunteers were exposed to barbecue smoke in outdoor air while at rest for 1.5 h, using a repeated-measures design. Major air pollutants were monitored. High particulate matter (PM_2.5) levels were observed during barbecue sessions, with averages ranging from 553 to 1062 μg/m3. Short-term exposure to air pollutants emitted from barbecuing was associated with a mild respiratory response in these healthy young adults, including prolonged “increase in nasal IL8 without a change in lung function and other measured inflammatory markers.” The results suggested possible prolonged respiratory inflammation, due to short-term exposure to barbecue fumes.

The toxic products of BBQ smoke

Specifically, the types of emitted VOCs that give rise to the above-mentioned health issues are as follows. Mencarelli A, et al. [13] reviewed those toxic products associated with BBQ smoke. Included among the various pollutants were, as mentioned, Particulate Matter (PM), Polycyclic Aromatic Hydrocarbons (PAHs), Carbon Monoxide (CO), Carbon Dioxide (CO2), Volatile Organic Compounds (VOCs), trace metals, and other minor pollutants [4,6,7,14,15]. For our purposes in this review focus will be primarily on the VOCs because that is the one set of pollutants that we have a possible chance of cleaning, filtering or scrubbing while leaving behind the desirable components of BBQ smoke, the nature of which is to be discussed below. Some of the more important chemicals in BBQ smoke are:

Benzene: a carcinogenic compound that can cause leukemia and other blood disorders¹.

Formaldehyde: a toxic gas that can irritate the eyes, nose, throat, and lungs. It can also cause allergic reactions and asthma attacks1.

Acrolein: a harmful chemical that can damage the lungs and increase the risk of respiratory infections1.

Toluene

The emission concentrations of a number of aromatic Volatile Organic Compounds (VOCs) and carbonyl compounds were quantified by Kabir E, et al. [7] during the combustion of commonly used barbecue charcoal. The concentrations of VOC and carbonyls were determined by gas chromatography coupled with thermal desorption and the HPLC method, respectively. The analysis of VOC emission concentrations showed that toluene (116 ± 444 ppb) was the most abundant. That does not mean, of course, that it is the most potentially pathologic VOC. They are all potentially dangerous to some extent.

Particulate matter is a problem too

Particulate matter is also a major issue for the reasons mentioned above. As mentioned, PM_2.5 consists of small invisible particles, with diameters ≤ 2.5 μm. Even ambient PM_2.5 is potentially dangerous, because the inhalable particles, and the toxic compounds they carry, do not only cause eye, nose, throat, and lung irritation. They can penetrate the lung tissue, and pass into the bloodstream, compromising the functioning of the heart and lungs, and exacerbating respiratory and cardiopulmonary disorders [16]. Table 2 shows that PM_2.5 concentrations greater than 35.5 μg/m3 are a health concern.

Fuel choice and other factors

Many fuel choices have been used for the American BBQ grill. Most people have a preference based on taste and ease of use. However, different grilling fuels are associated with different impacts on air quality and health.

• Charcoal: is usually accredited with having the most harmful emissions including such VOCs, PMs and PAHs.
• Smoking wood e.g. wood chips: Smoking woods such as hickory and mesquite emit significant amounts of harmful substances when burned. The EPA has issued many warnings in this regard and has been quick to mention specific pollutants such as benzene, acrolein and formaldehyde associated with them.
• Natural gas and propane- Widely considered as an ideal alternative to charcoal grilling, natural gas and propane release far less PM than charcoal and smoking woods. In addition, they can easily be turned off when grilling is finished.
• Charcoal grilling is responsible for air pollution that extends past the boundaries of a single barbecue and can negatively impact public health [1].

Mitigation strategies to date to reduce health risks

Here we review the progress e.g. carbon scrubbers to date and the steps that have been taken to solve the health concern for BBQ smoke in general. This does not include BBQ smoke associated with the American backyard BBQ grill. Some studies have involved BBQ smoke involving restaurants or sometimes kitchens. Typically, we are talking about carbon scrubbers Mohamed F, et al. [18] the smaller version of those used for industrial purposes to scrub smoke of pollutants.

When smoke is passed through an activated carbon filter, VOCs are selectively removed by adsorption. defined as the process of a gas or liquid adhering to the surface of a solid substrate, as opposed to absorption, which is the process of a gas or liquid penetrating a solid substrate [19]. When smoke is passed through an activated carbon filter (scrubber), the VOC adsorption efficiency may be as high as 99%; however, the proportion of VOC remaining in the smoke after filtration depends on the physio-chemical properties of the filter [20,21]. Different types of activated carbon filters may also selectively remove PM_2.5 [22].

Mencarelli A, et al. [13] reviewed some of these strategies. They noted that charcoal grilling emissions contain a wide range lump charcoal of pollutants including CO, CO2, NOx, PM, PAHs, VOCs, and trace metals. Compared to domestic emissions, charcoal grilling restaurants appeared to be a major source of air pollutants affecting both indoor and outdoor air quality. They found that workers in restaurant settings, exposed to cooking fumes for several hours, are particularly vulnerable to these health risks, but even short exposure can lead to health problems. Mitigation strategies involved different approaches, including the use of high-quality charcoal, using grilling equipment designed to reduce emissions, ensuring proper ventilation, and using abatement systems were all employed. Implementing these strategies guarantees safer grilling conditions while effectively reducing the adverse impacts of charcoal combustion on human health [23].

The factor of distance

At first glance the notion that barbecuing is typically outdoors might give some sense of security in that the pollutants will dilute out with distance or the wind may blow it all away. But this needs confirmation [13]. Despite the fact that indoor pollution poses a greater risk of exposure to air pollutants, outdoor charcoal grilling also represents a potential risk to users close to the stove. The distance from the barbecue is a very significant parameter to reduce exposure to harmful compounds. Lenssen ES, et al. [12], noticed that increasing the distance reduced the concentrations of PM_2.5, but also reduced particle number concentrations, and black carbon. Based on the research conducted by Lao, et al. [4], it was also observed that increasing the distance from 2 to 10 m resulted in decreased exposure to PM-bound polyacrylic hydrocarbons. The detected samples at 2 m and 10 m ranged between 0.43-3.27 µm and 0.601.56 µm, respectively. The findings paralleled that study conducted by Wu CC, et al. [24], which observed lower concentrations of PM₁₀, and PM_2.5, in smoke located at a distance of 10 m compared to 2 m. As the distance increases, the influence of environmental factors on the concentration of pollutants becomes more pronounced. Distancing is not the complete solution but is definitely a major factor.

A hypothesis worth testing

The problem with all mitigation strategies to reduce the harmful effects of BBQ smoke is that ridding the smoke also rids the very ingredients that make it desirable for cooking. The American BBQ user is not likely to be happy with that. Syringol, which unfortunately is a VOC, provides the desirable olfactory aspect and Guaiacol provides the desirable taste aspect when it reaches the food. Clearly all attempts to reduce BBQ smoke in its entirety are not helping the cooking process. Ideally, a means is needed to differentially remove the unwanted VOCs if such is possible. That process has never been done before. Of great interest, however, is that there is a theoretical possibility for that to happen. Both Syringol and Guaiacol are phenols which are polar molecules. Not all VOCS are polar. They are non-polar as is activated carbon which has a proclivity to remove non-polar molecules as opposed to polar molecules. It is not unreasonable to hypothesize, therefore, that when tested, it may be found that the desirable aromatic VOCs are not adsorbed anywhere near that of the other VOCs making it possible to at least clean the worst VOC parts of the BBQ smoke.

Components of BBQ smoke that are desirable

Vincente ED, et al. [4] do recognize the Syringol/Guaiacol dilemma, i.e. they are toxic yet desirable. Syringol is an organic compound with the formula HO(CH₃O)₂C₆H₃. It is beneficial, because it causes the sweet smoky smell of barbecued food, whereas Guaiacol contributes mainly to the taste of smoked food. Specifically, Guaiacol is an organic compound with the formula C6H4(OH)(OCH3). It is a phenolic compound containing a methoxy functional group.

Guaiacol appears as a viscous colorless oil, although aged or impure samples are often yellowish. It occurs widely in nature and is a common product of the pyrolysis of wood. Guaiacol is usually derived from guaiacum or wood creosote. It is produced by a variety of plants. It is also found in essential oils from celery seeds, tobacco leaves, orange leaves, and lemon peels. The compound is present in wood smoke, resulting from the pyrolysis of lignin.

Of great interest for the American BBQ griller, when meat is cooked over direct heat, such as on a barbecue, several chemical reactions occur: 1) water evaporation: Initially, water near the meat’s surface boils off due to the high temperature. Once the surface is dry, the next steps unfold; 2) the proteins and sugars on the meat’s exterior undergo a reaction called the Maillard Reaction [25]. This process produces a range of flavorful compounds, including

Guaiacol and Syringol, which contribute to the irresistible taste and smell of barbecued meat.

Helpful information from existing adsorption studies

Differential adsorption studies for Syringol and Guaiacol have not been done but the kinetics of some phenols have been investigated. For example, Ma X, et al. [26] studied the adsorption/desorption behaviors of semi-volatile organic compounds (SVOCs: 1,2,3,4 Tetrachlorobenzene (TCB) and phenol) in vapor phase by activated carbon. Investigations showed that at 100-160 ◦C, the adsorption capacities of TCB and phenol on AC were in the range of 176.6-342.0 mg/g and 24.0-66.4 mg/g, respectively. Increasing the temperature inhibited the SVOCs adsorption. TCB tended to be adsorbed on the activated carbon surface by monolayer, whereas the phenol was multilayer adsorption. Studies like this suggest we can anticipate Syringol/Guaiacol adsorption that is different than other VOCs, but we have to be weary of factors such as temperature which may significantly alter the adsorption in a direction contrary to the desired differential correction.

A differential adsorption pilot experiment

A pilot experiment was performed to exhibit the predicted differential adsorption. Smoke generated from an ordinary BBQ grill was allowed to be scrubbed by a high concentration that removed all VOCs including Syringol. The Syringol was monitored indirectly by an olfaction test. A 50% reduction in carbon concentration resulted in a dramatic differential reduction of the VOCs.

Apparatus

A standard small Weber BBQ was altered to contain an outflow hose on the top that split into 2 long (6 feet) hoses leading away from the BBQ to 2 TVOC/PM 2.5 meters (BLATN BR-smart 128s) (Figure 1). A tight seal was obtained at all junctions. A cork filled portal was constructed along the course of the hoses for the olfactory (smell) test. To monitor the grill temperature a thermometer was added to the hood.

Figure 1: Standard BBQ grill. A 2 cm diameter, steel lined, valve-controlled hose exited the lid. It split into two components and extended for a distance of approximately 6 feet. Along the course of one hose, one of 2 scrubbers was inserted. In addition, a 2 cm portal for the olfaction test was created and corked. The meters were located up against the ends of the hoses which fit tightly into the entry portion of the meters.

Carbon scrubbers of different strengths were made by filling a 30 x 7.5 cm plastic cylinder with Australia Virgin activated carbon. The 100% scrubber was filled with carbon only. The 50% scrubber was filled with carbon in one half and glass marbles in the other. The control (0% scrubber) was a plastic cylinder filled with marbles only. One hose incorporated the control (0% concentration – marbles only) and the other hose incorporated one of the 2 scrubbers. The fuel type to create the smoke was from wood chips (Weber - oak). Wood chips were chosen instead of charcoal because it produces the most smoke and is likely to produce the most smoke and Syringol. Guaiacol was not tested simply because it would be a more onerous task to cook food and subject it to a tasting protocol.

Participants

10 adult volunteers were asked to smell samples of the smoke coming from the corked portal. Upon cork removal they had approximately 2 seconds to smell the smoke. They were asked to rank the pleasantness (from 1-5 using a Likert scale in which 1 = odorless or unpleasant; 5 = strong pleasant BBQ type smell). No mention was made to the participants as to what scrubber was being tested.

Protocol

Four sets of 5 trials were run for each of the two scrubber conditions (100%; 50%) for a total of 80 trials. These sets were all randomized. The 0% concentration scrubber acted as the control. The BBQ grill temperature was kept between 200 -350 degrees F. (frequently used when barbecuing). One of two scrubbers was incorporated into one hose. The other hose remained as the control. First baseline normal air ambient values were obtained, then total volatile organic compound (TVOC) values were noted every minute. The two meters automatically stored the results. To minimize the issue of variability between the meters, they were frequently switched. TVOC values that went beyond the meter’s limit of 10.0 were given a value of 10.0 and this did occur in some cases. When the TVOC value plateaued in response to the arrived smoke, the 2 second olfaction test was performed.

Selective (Differential) adsorption of TVOC and syringol

To identify differential adsorption by the activated carbon we need to be able to compare TVOC (total volatile organic compound) scores to olfactory scores. Therefore, it was important to convert the units to ‘percent remaining.’ Figure 2 notes the percent remaining TVOC and percent remaining Syringol as a function of the scrubber concentration. When there was no scrubber (i.e., 0 %) very high values (median of 10 mg/m^3) were noted. This is far beyond the known hazardous level of 1.0 mg/m^3. By contrast, a 100% scrubber was able to reduce the TVOCs to a median of 0.01 mg/m^3. This value is below the hazardous level as seen in table 1 [27].

Figure 2: The percent remaining Syringol and TVOC is plotted as a function of the scrubber concentration. Note that when there is a maximal scrubber (100%) or no scrubber (0%) a differential is not seen. However, at 50% concentration approximately 65% of the Syringol remained in comparison to 15% of the TVOC. This is statistically significant at p < 0.05 (Mann-Whitney U).

However, partial scrubbers had a very distinct effect. For example, at 50% concentration the percent remaining Syringol was approximately 65% in comparison to a remaining TVOC of 15%. That is an approximate fourfold difference. As expected, this effect was not seen with scrubbers of 100% concentration. The results were statistically significant at the p < 0.05 level using the Mann-Whitney U test (Figure 2).

The results are encouraging in that this differential adsorption process does seem to provide a potential to rid much of the toxic VOCs. It is not certain how much of the PAHs would be adsorbed at the level needed to retain Syringol and Guaiacol. However, that would be the subject of future investigations. More important those investigations would need to confirm this pilot study. Ideally, Syringol should be measured with gas chromatography or spectroscopic means. The major limitation of this pilot study with the olfaction test is its inaccuracy. On the other hand, it is the best test for what we want to selectively preserve. No matter what the chromatographic numbers would be or turn out to be for Syringol, if the smell is not preserved the whole purpose of differential scrubbing would be for naught. One other important future investigation would be to see if a BBQ grill can actually be designed to scrub the smoke. There is much work to be done.

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