4.1 Introduction

As lasers vaporize and coagulate tissue, the thermal interactions can produce an aerosolized mixture of vapor, particulate debris, and smoke that is known as “plume.” The distinct odor is memorable to patients and those who use the lasers alike, and there are an increasing number of investigations into the laser plume and its impact on those who inhale it. In addition to inorganic irritating and mutagenic chemical matter, aerosolized organic particles (such as viral and bacterial matter) have been noted to be present in laser plume, depending upon the procedure. This chapter reviews the properties of laser plumes, their impact on human health, and protective strategies for those who perform and receive laser therapy.

Laser plumes are generated by laser treatments that ablate, vaporize, or heat their photothermolytic targets. Given their nature, ablative lasers such as CO₂ and Er:YAG will generate a plume as a consequence of direct tissue ablation. Nonablative lasers are also capable of creating laser plumes, depending on their therapeutic target and consequent generation of thermal energy. ^{1 , 2} Laser hair removal produces a plume as light is absorbed by melanin in the hair shaft and transformed into thermal energy, which results in the destruction of the follicular stem cells. The hair is consequently vaporized into plume as it combusts.

4.2 Inorganic Contents of the Plume

Studies of particulate matter within the laser plume closely followed after studies began to raise concerns about electrosurgical smoke. The majority of inorganic particulate matter in laser plume is composed of ultrafine particles (UFPs). ³ UFPs are defined as nanosized particulate matter less than 100 nm. As technology has become ubiquitous, so too have UFPs, which humans are exposed to in the environment regularly through driving, cooking, smoking, and operating household electrical devices. ⁴ Increased exposure to UFPs in particulate air pollution has been associated with increased mortality and cardiopulmonary disease. ^{5 , 6} The advent of modern technology has further brought about increased exposure to UFPs in activities of daily life, and nanotoxicological studies have highlighted the effects of UFPs on human health. When inhaled, UFPs are able to diffuse across respiratory and cutaneous epithelium, and their nanoscale size allows them to be taken up by a number of cells in the body, where they can exert oxidant and toxic effects. ⁷

There are a number of factors that influence the content and concentration of laser plumes (Table 4.1). Investigations have verified that the concentration of particulate matter in the air directly increases following laser procedures, and that the concentration of particulate matter decreases with time after the procedure and increased distance from the surgical site. ^{3 , 8} Compared to ambient concentrations, particulate concentrations in the procedure room increase during the procedure and remain elevated post-procedurally over an hour later. ⁹ UFP concentrations gradually drift back toward baseline levels as they diffuse into their surroundings, and their dissipation and peak concentration is heavily dependent on ventilation in the procedure room. ^{3 , 8 , 9} The duration of the procedure is the factor most closely associated with laser plumes. ⁹ Contact cooling with aqueous gel or lotion has been shown to reduce plume production when compared to cryogen devices for reasons that have not yet been elucidated. ^{9 , 10} It has been theorized that UFPs diffuse into and are retained by the coolant. In addition, the force of the cryogen spray as it is released is hypothesized to disperse UFPs. Increased laser energy has also predictably been linked to higher particulate concentrations. ^{3 , 9}

In vitro and in vivo studies of CO₂, Nd:YAG, and alexandrite lasers have characterized the gaseous particulate matter within the plume, which is composed of over 350 different identifiable compounds. ¹¹

The chemical contents of laser plumes have raised concerns about their carcinogenicity. As aforementioned, even ambient UFPs in the environment have been linked to carcinogenesis; in fact, particulate matter in outdoor pollution has been given a “1” rating by the International Agency for Research on Cancer, indicating a known carcinogen. Some of the chemicals found in higher concentrations within laser plumes include known carcinogens such as acetamide, acrylonitrile, benzene, butadiene, formaldehyde, naphthalene, propene, and styrene, among others. ^{11 , 12 , 13} In addition, some of the chemicals identified in plume have been linked to cancer, but without enough evidence for the International Agency for Research on Cancer to label the chemical a likely or known carcinogen. Taken together, the chemicals in the laser plume have been associated with a number of different malignancies. In particular, lung, urothelial, and hematologic malignancies seem to be quite common. ¹³ The close association with these malignancies in particular may likely be directly related to the small size of UFPs, as the chemicals are inhaled (where they contact respiratory mucosa), effectively enter endothelial cells as a result of their small size, spread to bone marrow, spleen, and lymph nodes, and are ultimately excreted into urine where they concentrate in the bladder. ⁷ It is unclear what the “safe” concentrations are for particulate matter. As an example, benzene, which is present in plume, has made its way to the public eye for its myelotoxicity and role in development of acute myeloid leukemia and acute nonlymphocytic leukemia. ¹³ Its extreme carcinogenicity has led to the United States Environmental Agency setting a maximum permissible level of 5 parts-per-billion, with a goal of 0 parts-per-billion. While it is clear that no amount of benzene is safe, such regulations do not exist on every chemical within laser plume, making it difficult to know when particulate matter has exceeded a threshold of safety. Ultimately, although laser plume is composed of a number of carcinogenic compounds in various concentrations, there are no human studies investigating the long-term carcinogenic effect of plume itself. That being said, surgical smoke from electrocautery, which has a similar composition to laser plume, has been demonstrated to be mutagenic in several studies. ¹⁴ Clearly, further study is merited.

In addition to carcinogenicity, the inorganic contents of the laser plume have both direct and indirect effects on cardiopulmonary function. In animal models, exposure to Nd:YAG plume reproducibly and invariably induces significant emphysematous change. ^{1 , 15} This finding has been reproduced with exposure to CO₂ plume. ¹⁶ Many of the chemicals in laser plume are also found in cigarette smoke, albeit in lesser concentrations. In cigarette smokers, hydrogen cyanide is estimated to account for 89% of cardiovascular potency, whereas acrolein has been estimated to account for about 97% of respiratory potency. ¹⁷ Cyanides (i.e., acrylonitrile) and acrolein are found heavily within the plume, and potentially account for the cardiopulmonary impact observed in animal models, as they do in human cigarette smokers. ¹¹ Cyanide toxicity leads to inhibition mitochondrial oxygen utilization, leading to cellular hypoxia.

Finally, an overwhelming number of chemicals in the plume are irritants to cutaneous, mucosal, and respiratory epithelium, and exposure to many can acutely cause malaise, dizziness, nausea, vomiting, and headache. ¹⁸ Outside of these acute mucocutaneous irritation and constitutional symptoms, there are no human studies that investigate the actual health effects of laser plume particulate matter.

4.3 Organic Contents of the Plume

In addition to harmful inorganic ultrafine particulate matter, there is aerosolized biologic material contained within laser plumes as well. Aerosolization of the human papilloma virus (HPV) is perhaps the most well-described biohazardous material within the laser plume. Early in-vitro and in-vivo studies of verrucae treated with CO₂ laser demonstrated that intact HPV DNA was readily detected in the plume of about 30 to 60% treated patients. ^{19 , 20} Aerosolization of the virus does not appear to be universal for every laser type, as HPV DNA has not been identified in the plume of patients with warts or papillomas treated with Er:YAG or KTP laser. While the virus is certainly detectable in the CO₂ laser plume, there have been a number of studies debating the true infectious potential of such aerosolized papillomavirus. This risk appears to increase when treating genital HPV, perhaps due to the propensity of the common causative HPV subtypes in genital warts (6 and 11) to also infect oropharyngeal mucosa. ²¹ CO₂ laser surgeons have a similar incidence of warts as the general population, yet have a higher incidence of anogenital, nasopharyngeal, and plantar warts. ²¹ There have been case reports of iatrogenic CO₂ laser-induced laryngeal papillomatosis, described in healthcare personnel who were extensively involved in treatment of anogenital condylomas. ^{22 , 23} In addition, there have been reports of HPV-positive tonsillar cancer occurring in laser surgeons without any other identifiable risk factors. ²⁴ Taken altogether, the evidence appears to support a very low, albeit very real risk of infection from the liberalized virus.

Bacterial cultures of laser plume following CO₂ resurfacing demonstrated growth of skin flora including Staphylococcus and Corynebacteria, ²⁵ implying that laser plume may contain viable bacteria. HIV proviral DNA has been detected in CO₂ laser plume; however, these findings have not been reproduced and simian immunodeficiency virus does not appear to be viable within plume. ^{26 , 27}

4.4 Protective Strategies

Given the mounting evidence that suggests actual and theoretical laser plume toxicity, it follows that protective strategies to minimize plume exposure are imperative.

As aforementioned, the peak concentration of UFPs and their rate of clearance are dependent on ventilation of the laser operating room. Dilution ventilation is a mode of ventilation that ventilates air from an entire building or area using exhaust fans. This is what many would consider to be standard ventilation for most office buildings. However, as previously mentioned, dilution ventilation is necessary, but not sufficient in removing UFPs from the operating room. ⁹ Thus, local exhaust ventilation is necessary in eliminating laser plume. Local exhaust ventilation extracts the contaminant as it is generated, keeping it from being widely aerosolized. In the setting of laser surgery, local exhaust ventilation typically refers to the use of smoke evacuators. In light of a paucity of data, guidelines on laser plume exposure reduction are typically extrapolated from studies on surgical smoke. It has been repeatedly demonstrated that use of smoke evacuators significantly reduces but does not totally eliminate surgical smoke. ²⁸ Guidelines often recommend keeping the smoke evacuator inlet nozzle within 2 inches of the site; however, keeping the inlet closer allows for more effective evacuation. In laser hair removal, the distance of the smoke evacuator from the source is directly proportional to the UFP concentration in the air. ⁹ Each smoke evacuator has inherent properties that impact its ability to remove UFPs from the air (Table 4.2). It is important to select a smoke evacuator with high filtration efficiency. Ultralow-penetration air filters are preferable and characteristic of most modern smoke evacuators. These filters have an efficiency of 99.999% for particles 12 nm and smaller. As UFPs compose the majority of laser plume, a filter that captures these small particles is essential. ²⁹ Guidelines for minimum airflow of electrocautery smoke have been described; however, it is important to note that the density of the laser plume is not necessarily comparable to that of electrocautery smoke, and so these guidelines cannot be effectively extrapolated to laser plumes. As would be expected, higher minimum airflow of the smoke capture device allows for improved capture of UFPs. ³⁰ A capture of velocity of 100 to 150 feet per minute at the inlet nozzle is generally recommended for evacuation of laser plumes.

Given the previously discussed irritating effects of laser plumes on the ocular and respiratory mucosae, protective equipment is essential. Eye protection will prevent eye irritation from the UFPs in the plume. For the majority of laser procedures, laser safety goggles are required regardless to protect the eye from the laser beam. Masks are essential tools to prevent inhalation of the plume and its subsequent toxic effects. Not all masks have equivalent protective capability. The standard disposable surgical mask is capable of filtering 91.5% of particulate matter larger than 5,000 nm. ³¹ Given than UFPs (< 100 nm) make up the majority of the plume, the standard surgical mask provides insufficient respiratory protection. High-efficiency particulate air masks are capable of filtering particulate matter larger than 300 nm. The efficiency of the filtration masks is variable. Perhaps the most commonly used high-efficiency particulate air mask is the N95 respirator, named for its minimum 95% filtration efficiency. In actuality, the filtration efficiency of the N95 mask may be closer to 99.9%. ³¹ In face of the small particulate size characteristic of laser plume, the N95 mask or a similar high-efficiency particulate air mask is preferable to a standard mask. Proper fit of the mask is essential to its function. A mustache or beard can prevent proper fit. The mask should cover the nose and the mouth. So-called “laser masks” should be carefully examined for filtration efficiency and filtration size, as the name does not necessarily imply more respiratory protection.

As the properties and risks of the laser plume are further explored, further ways to prevent potential harm will likely be elucidated. As eluded to earlier in the chapter, contact cooling with aqueous gel appears to be a promising mechanism to suppressing laser plume. Until these methods are further developed, respirators, smoke evacuators, and effective ventilation systems remain the mainstay of laser plume hazard mitigation.