Tal Usvyatsky
Section C
Lit Review
Introduction
Toluene, a common industrial solvent, is widely used in products including nail
polishes, paint thinners, and many fuels (Myers, 2007). In addition to its popularity, the
high volatility of the substance creates a significant risk of accidental toluene inhalation.
Exposure to high concentrations of toluene through either unintentional or purposeful
inhalation is known to cause significant damage to the human central nervous system,
earning toluene its label of ‘neurotoxin’ (EPA, nd). In an effort to protect worker and
consumer safety around the chemical, several government agencies have set protective
limits on toluene exposure. However, many of these limits restrict only high exposures,
even though research suggests that even lower concentrations of toluene can cause
neurological harm (von Euler, 2000). Furthermore, some material cited by the Food and
Drug Administration (FDA) regarding toluene use in nail polish dates back to 1987 and
originates from an error-containing report (CIR, 1987).
Although toluene has been commonly researched in mice, Girardia tigrina,
colloquially known as brown planaria are a viable option for the study of toluene’s effects
on the central nervous system. Due to their regenerative abilities and their possession of a
central nervous system that is similar to that of many vertebrates, planaria are a popular
model for neurotoxicological research. Exhibiting a negative phototoxic response,
planaria can be trained to navigate a maze. Because they are able to learn and retain
memory of learned behaviors through truncation and head regeneration, planaria can be
used to study the effects of toluene in low concentrations.
Toluene
Toluene, or methylbenzene, is a clear, flammable, aromatic hydrocarbon liquid
consisting of a methyl group and a benzene carbon atom (Myers, 2007). Most commonly
produced through the distillation of crude oil, toluene is a popular solvent used in
industries ranging from adhesives to pharmaceuticals. The substance is a volatile organic
compound and a derivative of benzene (ATSDR, 2000).
Figure 1. The diagram shows the chemical structure of toluene with a benzene ring and methyl group and
seven carbon atoms bonded to eight hydrogen atoms (UCLA, nd).
Common toluene-containing products include paint, paint thinner, shellacs,
varnishes, dyes, and inks (Myers, 2007). Additionally, toluene can be found in nail
polishes, hair dyes, and other cosmetic products (Harbison, 2015).
Figure 2. The image shows the back of a bottle of nail polish and the chemicals that compose the nail
polish. In the middle-right of the label, it can be seen that the nail polish contains toluene.
Production of nylons, plastics, bottles, dyes, and polyurethanes also involves the use of
toluene (Harbison, 2015).
Exposure
Toluene exposure can occur through ingestion, skin contact, and most commonly
inhalation. The Environmental Protection Agency (EPA) found traces of toluene in
drinking water; however, the amounts were so minute that they did not pose a risk to
humans’ health (EPA, nd).
Individuals working in nail salons or using nail polish containing the substance at
home may be exposed to toluene through inhalation of vapors or contact of the nail polish
with the skin. Employees working with gasoline, paints, and lacquers are at an even
higher risk of toluene-induced effects, especially if they are exposed for long periods of
time. Workers in those industries may incur concentrations of up to 800 ppm.
Known Effects on Humans
Toluene toxicity primarily affects the central nervous system (CNS) for chronic
and acute exposures (EPA, nd). Acute exposure in humans often causes CNS dysfunction
and narcosis with symptoms of fatigue, sleepiness, headaches, and nausea. Humans with
chronic exposure to high levels of toluene have experienced CNS depression as well as
irritation of the upper respiratory tract and eyes, sore throat, dizziness, and headache.
There is inadequate information to assess the carcinogenic nature of toluene (EPA, nd).
Other effects of toluene on the human body are conjunctivitis, keratitis, staggering
gait, tremor, euphoria, disorientation, hallucinations, CNS depression, ventricular
arrhythmias, chemical pneumonitis, respiratory depression, nausea, vomiting, and
electrolyte imbalances (Harbison, 2015).
Toluene is often purposefully inhaled by adolescents for its intoxicating effects
(Beckley & Woodward, 2011).Voluntarily inhalation of 2000 ppm toluene can induce
transient euphoric effects, with long-term consequences like persistent cognitive and
neurological harm (von Euler, 2000). Toluene abuse can adversely affect cognitive tasks
such as attention, behavioral control, and risk assessment (Beckley & Woodward, 2011).
In a 1988 study, six chronic toluene abusers were studied using MRI and another abuser
was studied without MRI. Diffuse cerebral, cerebellar, and brainstem atrophy, loss of
differentiation between gray and white matter, and increased periventricular white matter
signal intensity could be seen in the brain imaging of six abusers. The seventh toluene
abuser suffered an acute toluene overdose that ultimately caused his death. Diffuse, illdefined myelin pallor was observed in the brain and nervous system. Increased water
content of the white matter and toluene-induced changes in myelin were offered as
possible explanations for the observed effects (Rosenberg, 1988).
Although the consequences of high concentrations of toluene are largely
undisputed, the effects of low concentrations of the chemical on neurological and
cognitive function leave room for debate. In one study, rats exposed to 80 ppm toluene
for four weeks showed decreased spatial learning in a Morris water maze test (von Euler,
2000). However, another study showed that even at exposures as high as 600 ppm,
toluene did not cause significant damage to memory in rats (Miyagawa, 1995).
Laws and Regulations
Several government agencies contribute to standards and recommendations for
toluene exposure. The National Institute for Occupational Safety and Health (NIOSH)
created a recommended exposure limit for toluene in general industry, and its lawmaking counterpart, the Occupational Safety and Health Administration (OSHA), made
federal limits that must be obeyed by all employers in the United States. NIOSH and
OSHA also outline precautions and measures necessary for worker safety. The Food and
Drug Administration (FDA) cites the Cosmetic Ingredient Review (CIR) for acceptable
toluene use in nail polish. However, the reports currently have unaddressed flaws in their
content.
OSHA, an agency with legislative authority, set a permissible exposure limit
(PEL) of 200 parts per million (ppm). The PEL is a time-weighted average over a typical
8-hour work shift. Within the PEL, the acceptable ceiling concentration of toluene is 300
ppm, and the acceptable peak above the maximum concentration is 500 ppm for a
duration of no more than ten minutes. (OSHA, 1997). However, NIOSH recommends
limiting exposure to 100ppm. (NIOSH, 2011). However, the vast majority of PELs are
more than 40 years old and completely inadequate, warranting a reevaluation (Marvin
Lewiton, personal communication, December 11, 2015). The current toluene PEL,
especially, is not adequately protective (Marvin Lewiton, personal communication,
December 11, 2015).
TABLE Z-2 (OSHA, 1997)
Acceptable maximum peak above the
Substance
Benzene(a) (Z37.40-
8-hour time
Acceptable
acceptable ceiling concentration for an
weighted
ceiling
8-hr shift
average
concentration
10 ppm
25 ppm
Concentration
50 ppm
1969)
Maximum
duration
10
minutes.
Beryllium and beryllium 2 ug/m(3)
5 ug/m(3)
25 ug/m(3)
compounds (Z37.29-
30
minutes.
1970)
Cadmium
0.1 mg/m(3)
0.3 mg/m(3)
Cadmium dust(b)(Z37.5- 0.2 mg/m(3)
0.6 mg/m(3)
.........
fume (Z37.5-1970)
(b)
1970)
Carbon disulfide
20 ppm
30 ppm
100 ppm
(Z37.3-1968)
Carbon tetrachloride
(Z37.17-1967)
30
minutes.
10 ppm
25 ppm
200 ppm
5 min. in
any 3 hrs.
Chromic acid and
..........
1 mg/10 m(3)
20 ppm
30 ppm
50 ppm
5 minutes.
50 ppm
100 ppm
200 ppm
5 min. in
chromates (Z37-71971)(c)
Ethylene dibromide
(Z37.31-1970)
Ethylene dichloride
(Z37.21-1969)
Fluoride as dust
any 3 hrs.
2.5 mg/m(3)
...........
.........
...........
...........
.........
3 ppm
..........
.........
..........
20 ppm
50 ppm
(Z37.28-1969)
Formaldehyde: see
1910.1048
Hydrogen fluoride
(Z37.28-1969)
Hydrogen sulfide
(Z37.2-1966)
10 mins.
once only if
no other
meas. exp.
occurs.
Mercury (Z37.8-1971) ..........
1 mg/10m(3)
........
Methyl chloride
200 ppm
300 ppm
100 ppm
(Z37.18-1969)
5 mins. in
any 3 hrs.
Methylene Chloride:
see 1910.1052
Organo (alkyl) mercury 0.01mg/m(3) 0.04 mg/m(3) .........
(Z37.30-1969)
Styrene (Z37.15-1969) 100 ppm
200 ppm
600 ppm
5 mins. in
any 3 hrs.
Tetrachloroethylene
100 ppm
200 ppm
300 ppm
5 mins. in
any 3 hrs.
Toluene (Z37.12-1967) 200 ppm
300 ppm
500 ppm
10 minutes
Trichloroethylene
200 ppm
300 ppm
5 mins. in
(Z37.19-1967)
100 ppm
any 2 hrs.
Some state-level OSHA programs have set stricter limits on toluene exposure.
California has set a PEL of 10 ppm (OSHA, nd), which is also recommended by OSHA.
Additionally, OSHA details the necessary measures that must be taken by
employers to ensure worker safety surrounding toluene. Workers must be provided with
proper equipment, including NIOSH approved respirators with organic vapor cartridges
Commented
(OSHA, nd). This is to prevent unwanted inhalation of toluene fumes. In addition,
protective gloves must be worn (OSHA, nd).
CIR produced a report in 1987 regarding the safety of toluene in nail polish. In
this report, they determined that toluene was safe in nail polish in concentrations no
greater than 50 % (Liebert, 1987). These findings were reviewed in 2005; however, the
CIR Expert Panel elected not to reopen the safety assessment on toluene. The 2005
review evaluated more recent research on toluene, and deemed the newly discovered
adverse effects inapplicable because they occur at concentrations higher than those used
in nail polish (CIR, 2005).
However, the older report contains errors in fundamental chemistry, specifically
toluene’s solubility in water, calling into question the credibility of content of the report.
The 2005 reassessment does not investigate applicable information about toluene; rather,
it dismisses that which is inapplicable, essentially leaving the 1987 findings in effect.
These findings remain the basis of FDA documentation regarding toluene’s use (FDA,
2013). Additional research about lower concentrations of toluene is needed in order to
further evaluate safety of toluene in nail polish as well as other products.
Planaria
Planaria, of the class Turbellaria and phylum Platyhelminthes, are free-living
flatworms, mostly found in fresh-water. The regenerative abilities and ample supply of
adult stem cells in planaria have fascinated biologists for a long time (Rink 2012).
Anatomy and Physiology (G. Tigrina)
Planaria have bilateral symmetry, dorsal and ventral surfaces and a rostrocaudal
axis with a tail and a head with light-sensitive eyespots. (Buttarelli, 2008). A mouth with
a sometimes protruding pharynx can be found on the ventral side. The flatworms usually
measure 3 to 15 mm and can range in color. (Encyclopedia Britannica, nd). However, G.
Tigrina is brown, giving it its colloquial name: brown planaria.
Containing reproductive organs of both sexes, the flatworms are simultaneous
hermaphrodites; however, they are also capable of asexual reproduction, which can be
induced by cutting the planaria (Encyclopedia Britannica, nd).
Figure 3. Planarian anatomical features are depicted. (“GeoChemBio”, nd).
Planarian Central Nervous System
Planaria have a brain, longitudinal nerve cords, and peripheral nerve plexuses. A
commissure, or broad connection, joins two cephalic ganglia in the anterior part of the
animal, composing its brain. The brain also includes three to five pairs of longitudinal
nerve cords, which are connected by transverse commissures. Additionally, smaller,
lateral nerves extend from the cords (Lentz, nd).
Because of their shape, dendritic spines with synaptic boutons, axon, expression
of neural proteins similar to those of vertebrates, and fairly low spontaneously generated
electrical activity, neurons in planaria more closely resemble vertebrates than
invertebrates (Buttarelli, 2008).
Planaria and Neurotoxicology
Many neurotransmitter-receptor systems are present in planarians, and numerous
studies reveal distinct behavioral patterns after exposure to neural-transmission-altering
drugs. Consistently, planarians have been shown to be effective models for
neurotoxicology of the dopamine neurons and evaluation of certain drugs (Buttarelli,
2008).
Regeneration
Planaria are often used in experiments to study the process of regeneration,
because of their ability to regenerate lost parts (Encyclopedia Britannica, nd). A
transverse cut can induce regeneration such that the head grows a new tail and the tail
grows a new head.
Figure 4. Figure shows stages of planarian regeneration after a transverse cut (Gentile, 2011).
The planaria resulting from the original truncation are morphologically identical
to the original planaria. This phenomenon further validates drug testing in planaria
(Oviedo & Levin, 2008).
Learning and Memory
The systems that allow planaria to regenerate their brains and bodies cause
planaria to be widely researched. Planaria are also capable of long-term memory,
allowing regeneration and memory to be studied simultaneously. A training system for
planarian learning was developed and then used to teach planarian flatworms. The
planaria showed the ability to learn and their memory persisted for fourteen days. It was
also shown that decapitated planaria retained memory after regenerating a new head. For
this reason, planaria are an ideal model for learning and memory (Shomrat & Levin
2013).