R E S EAR CH Open Access

Thoracic and respirable particle definitions for

human health risk assessment

James S Brown1*, Terry Gordon2, Owen Price3 and Bahman Asgharian4

Abstract

Background: Particle size-selective sampling refers to the collection of particles of varying sizes that potentially

reach and adversely affect specific regions of the respiratory tract. Thoracic and respirable fractions are defined as

the fraction of inhaled particles capable of passing beyond the larynx and ciliated airways, respectively, during

inhalation. In an attempt to afford greater protection to exposed individuals, current size-selective sampling criteria

overestimate the population means of particle penetration into regions of the lower respiratory tract. The purpose

of our analyses was to provide estimates of the thoracic and respirable fractions for adults and children during

typical activities with both nasal and oral inhalation, that may be used in the design of experimental studies and

interpretation of health effects evidence.

Methods: We estimated the fraction of inhaled particles (0.5-20 μm aerodynamic diameter) penetrating beyond the

larynx (based on experimental data) and ciliated airways (based on a mathematical model) for an adult male, adult

female, and a 10 yr old child during typical daily activities and breathing patterns.

Results: Our estimates show less penetration of coarse particulate matter into the thoracic and gas exchange regions

of the respiratory tract than current size-selective criteria. Of the parameters we evaluated, particle penetration into the

lower respiratory tract was most dependent on route of breathing. For typical activity levels and breathing habits, we

estimated a 50% cut-size for the thoracic fraction at an aerodynamic diameter of around 3 μm in adults and 5 μm in

children, whereas current ambient and occupational criteria suggest a 50% cut-size of 10 μm.

Conclusions: By design, current size-selective sample criteria overestimate the mass of particles generally expected to

penetrate into the lower respiratory tract to provide protection for individuals who may breathe orally. We provide

estimates of thoracic and respirable fractions for a variety of breathing habits and activities that may benefit the design

of experimental studies and interpretation of particle size-specific health effects.

Keywords: Size-selective sampling, Fine and coarse particles

Background

It has long been recognized that the regional pattern of

particle deposition in the respiratory tract affects the

pathogenic potential of inhaled aerosols. For example,

Morgan [1] concluded that respirable dusts likely caused

pneumoconiosis and silicosis in coal miners, whereas a larger size fraction caused bronchitis and obstructive changes

in pulmonary function. Sampling the total air concentration of particulate matter (PM) provides a crude estimate

of exposure that may not correlate with observed health

effects if the risk is associated only with those particles that

may enter the thorax or penetrate beyond the ciliated airways. The concept of size-selective particle sampling has

been employed as a means for effectively sampling the

particle sizes associated with specific pathologic outcomes

(e.g., the respirable fraction with parenchymal disease). If

an environmentally or occupationally related particle is

recognized to only affect the gas-exchange region of the

lung, then a sampling strategy that only collects the respirable fraction of airborne PM is preferable to sampling total

suspended particulate (TSP) or the thoracic fraction.

The human respiratory tract can be divided into three

main regions based on size, structure, and function,

namely, the head, tracheobronchial region (also known as

* Correspondence: Brown.James@epa.gov 1

National Center for Environmental Assessment, U.S. Environmental

Protection Agency, MD B243-01, Research Triangle Park, Raleigh, NC 27711,

USA

Full list of author information is available at the end of the article

© 2013 Brown et al.; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative

Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and

reproduction in any medium, provided the original work is properly cited.

Brown et al. Particle and Fibre Toxicology 2013, 10:12

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the conducting airways), and the gas-exchange region (also

known as the parenchymal, alveolar, or pulmonary) region.

Size-selective sampling is intended to help discern the

amount of aerosol expected to be available for deposition

in a region. Most sampling conventions have been defined

in terms of particle penetration into respiratory regions rather than the expected particle deposition or dose to regions. Specific definitions used herein, adopted from the

European Committee for Standardization (CEN), are [2]:

 Inhalable fraction – the mass fraction of total

airborne particles which is inhaled through the nose

and mouth.

 Extrathoracic fraction – the mass fraction of inhaled

particles failing to penetrate beyond the larynx.

 Thoracic fraction – the mass fraction of inhaled

particles penetrating beyond the larynx.

 Respirable fraction – the mass fraction of inhaled

particles penetrating to the unciliated airways.a

The above definitions are stated in terms of a mass

fraction. Relative to total airborne particles, the particle

size having 50% penetration for the thoracic and respirable fractions are 10 μm and 4.0 μm (all particle sizes

are aerodynamic diameter unless expressed otherwise),

respectively [2,3]. These criteria were specifically developed for workplace atmospheres. Since particles must

generally become deposited to exert biological effects,

these conventions, based on regional exposure (i.e., particles penetrating into a region of the respiratory tract),

are conservative by design in that they overestimate the

amount of inhaled material that becomes deposited and

thereby available to induce an effect.

In 1985, the American Conference of Governmental Industrial Hygienists (ACGIH) recommended particle sizeselective sampling in setting threshold limit values for occupational exposures [4].b The ACGIH specifically considered

a reference worker (weight, 70 kg; height, 175 cm) breathing orally while engaged in light activity (minute ventilation,

21.75 liters/min). Criteria were established for Inspirable

(now Inhalable), Thoracic, and Respirable Particulate Mass

that were intended to be protective against materials that

were considered hazardous when deposited anywhere in

the respiratory tract, anywhere within the lungs, and in the

gas-exchange region, respectively. These criteria were based

on exposure of a respiratory tract region (based on particle

penetration into that region), not particle deposition in a respiratory tract region. The ACGIH committee recognized

uncertainty related to individual biological variability in respiratory health status, breathing patterns (rate and route),

and airways structure as well as differences in work rates,

all of which can cause differences in inhaled aerosol deposition and dose. Facing these uncertainties, the committee

afforded extra protection to exposed workers by over

representing the true penetration of particles into regions of

the respiratory tract as illustrated in Figure 1 [4].

Size-selective sampling has also been employed by the U.

S. Environmental Protection Agency (EPA) in setting the

national ambient air quality standards (NAAQS) for particulate matter (PM). In 1987, the EPA changed the indicator for PM from TSP (effectively an aerodynamic cut-size

varying from 25 to 40 μm, depending on wind speed and

direction) to PM10 (particles with a nominal mean aerodynamic diameter ≤ 10 μm) [8]. Consistent in concept with

the ACGIH thoracic particle fraction, PM10 delineates a

subset of inhalable particles (referred to as thoracic particles) that are thought small enough to penetrate to the

thoracic region (including the tracheobronchial and alveolar

regions) of the respiratory tract.c In 1997, the EPA extended

size-selective sampling to include fine particles indicated by

PM2.5 (particles with a nominal mean aerodynamic

Figure 1 Thoracic particulate mass fraction criteria (relative to

total airborne particles) for size-selective sampling. Individual

data points are observed human head penetration efficiency during

oral inhalation for an inspiratory flow rate of 43.5 L/min, i.e., light

exercise [5-7]. As stated by ACGIH [3], the sampling criterion is offset

to the right of experimental data to overestimate the amount of

exposure to the lower respiratory tract, i.e., the lungs, and

correspondingly to provide a greater level of protection for exposed

workers. From ACGIHW, Particle Size-Selective Sampling in the

Workplace, Report of the ACGIHW Technical Committee on Air

Sampling Procedures. Copyright 1985. Reprinted with permission.

Courtesy: Dr. Otto G. Raabe.

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diameter ≤ 2.5 μm) and retained PM10 as the indicator for

thoracic coarse particles [9]. The selection of PM2.5 by the

EPA was mainly to delineate the atmospheric fine (combustion derived, aggregates, acid condensates, secondary aerosols) and coarse (crustal, soil-derived dusts) PM modes and

for consistency with community epidemiologic health studies reporting various health effects associated with PM2.5.

With consideration to the PM NAAQS, Miller et al. [10]

also specifically recommended a particle size cut-point of ≤

2.5 μm as an indicator for fine PM based on consideration

of particle penetration into the gas-exchange region and

the delineation of the fine and coarse particle modes.

Most recently, the International Organization for Standardization (ISO) has released recommendations for sampling conventions based on particle deposition (rather

than exposure) in adult males and females engaged in activities of sitting, light exercise and heavy exercise as specified in Table 1 [11]. The ISO estimates of deposition were

determined using the International Commission on Radiological Protection (ICRP) human respiratory tract model

[12]. These new ISO conventions [11] are not considered

further herein as current sampling conventions for occupational and non-occupational settings remain dependent

on the probability of particle penetration rather than deposition in specific regions of the respiratory tract.

Conceptually, size-selective sampling better characterizes

PM exposure to regions of the respiratory tract and thereby

affords more appropriate avenues for protection of exposed

populations than TSP. Such a simple concept is not, however, without ambiguity in definitions and debate over appropriate sampling approaches. For example, the definition

for the thoracic fraction specifies particles “penetrating

beyond the larynx,” whereas the ACGIH thoracic convention for sampling (Figure 1) clearly and intentionally overestimates the fraction of large particles penetrating into the

thoracic region to afford extra protection of occupationally

exposed individuals. The purpose of this paper is to provide

realistic estimates of thoracic and respirable particle fractions for adults and children that may be used in the design

of experimental studies and interpretation of health effects

evidence.

The ICRP human respiratory tract model [12] was used

to estimate particle penetration through the extrathoracic

(ET) airways. The ICRP predictive equations for ET deposition are based on experimental measurements in humans.

Although also based on human data, the ICRP model was

not used to estimate penetration through the tracheobronchial (TB) airways due to its reliance on measurements of

particle clearance from the TB airways and the ability to

target particle deposition into the ciliated airways. That is,

much of the available regional deposition data for the TB

and alveolar regions have been obtained from experiments

with radioactively labeled, poorly soluble particles or by use

of aerosol bolus techniques (see Sections D.9.2 and E.5.3 of

Ref [12]). Aerosol bolus (40 ml volume of 3.5 μm particles)

inhaled to a very shallow lung volume (70 ml, ~75% of

phase I inert gas washout) by healthy adults (10 M, 6 F;

20-43 yrs of age) show preferential left lung deposition and

23% retention at 48 hrs [13]. This suggests slow TB airway

clearance and/or some penetration into the alveolar region.

Given the above, coupled with uncertainty related to slow

TB clearance [e.g., 14,15], we utilized the publicly available

multiple path particle dosimetry (MPPD; ver 2.1, © 2009)

model to estimate penetration through the TB airways.

Methods

Once particles have entered the respiratory tract via the

nose or mouth, the primary factors affecting particle penetration into the lower respiratory tract (i.e., beyond the larynx) are airways size and structure, breathing pattern (flow

and volume), route of breathing (nose vs. mouth), and inhaled particle size. With regard to particle size, we have

considered particles whose deposition is governed by their

inertial properties, i.e., ≥ 0.5 μm. Breathing patterns vary

mainly by sex, age, and activity. Table 1 provides the breathing patterns, subject groups, and activity patterns from the

ICRP [12] model that were used in our assessment.

Based on our comparison of the ICRP model [12] to

more recent data provided by Brochu et al. [16], the daily

ventilation rates and activity patterns provided in Table 1

overestimate typical daily ventilation rates. Table 2 provides daily ventilation rates (5th, 50th, and 95th percentiles)

reported by Brochu et al. [16]. The daily ventilation rates

from the ICRP [12] model’s recommended time budget

roughly correspond to the highly active 95th percentile

(see Table 1 vs. Table 2). To assess the effect of daily

Table 1 Ventilatory and activity patterns for adult males,

adult females, and a ten year-old child

Sleeping Sitting Light Heavy

Exercise Exercise

Adult Male VT (mL) 625 750 1250 1920

Sedentary worker f (min-1) 12 12 20 26

t (hr) 8.5 5.5 9.75 0.25

Vdaily (L/day) 3825 2970 14625 749

Adult Female VT (mL) 444 464 992 1364

Sedentary worker f (min-1) 12 14 21 33

t (hr) 8.5 5.5 9.75 0.25

Vdaily (L/day) 2717 2144 12187 675

Child (10 yrs) VT (mL) 304 333 583 752

Male or Female f (min-1) 17 19 32 45

t (hr) 10 4.67 9.33 0

Vdaily (L/day) 3101 1772 10447 0

VT, tidal volume; f, breathing frequency; t, time spent engaged in specific

activity; Vdaily, total volume inspired in 24 hr. Data are from ICRP [12] Tables

B15 and B16A-B for breathing and activity patterns, respectively.

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activity/ventilation rates on particle penetration into the

respiratory tract, we reduced the estimated time (given in

Table 1) that individuals spent engaged in light exercise

and correspondingly increased their time sitting to match

the daily ventilation rates in Table 2. To match the very

sedentary 5th percentile of daily activity, it was necessary

also to eliminate time spent by the adult female engaged

in heavy exercise and to attribute all of the child’s activity

to sleeping. Even when considering 100% of the child’s activity equivalent to sleeping, the ICRP [12] breathing pattern slightly overestimated (by 3%; 7.4 vs. 7.2 m3

/day) the

5th percentile daily ventilation rates of Brochu et al. [16].

Route of breathing varies with inspiratory flows and generally shifts from nasal to oronasal breathing at higher flows.

The ICRP [12] model characterizes breathing habit based on

Niinimaa et al. [17], who examined the route of breathing as

a function of activity in healthy adults (14 males, 16 females).

Eighty-seven percent of the subjects breathed through the

nose at rest and switched to oronasal breathing with exercise. These subjects were referred to as “normal augmenters.”

Thirteen percent breathed oronasally even at rest and were

referred to as “mouth-breathers.” For both of these breathing habits (i.e., normal augmenters and mouth-breathers,

we estimated the fraction of a breath passing through the

oral and nasal pathways from regression equations for oral

breathing in Figure 1 of the Niinimaa et al. study [17]. The

ICRP [12] model utilizes this same general approach, but

for each breathing habit and activity (i.e., sleeping, sitting,

etc.), the same fraction of oral breathing was assumed

applicable to all ages and both sexes.

In considering breathing habit, we differed from the

ICRP, in that we assumed the fraction of oral breathing to

differ between adult males and females as a function of

their minute ventilation rather than their activity level.

However, children tend to have a greater fraction of oral

breathing than adults at rest and during exercise [18,19].

Therefore, consistent with the ICRP, we assumed the fraction of the breath inhaled through the mouth (Fm) in the

child engaged in some specific level of activity was equal to

that of the adult male engaged in the same level of activity

despite the dramatically lower ventilation rates of the child.

We also considered recent breathing habit data not available for inclusion in the ICRP model [12]. Bennett et al.

[19,20] show a more gradual increase in oronasal breathing

than did Niinimaa et al. [12]. In addition to the normal

augmenter and mouth-breather breathing habits based on

the Niinimaa et al. [17] study, we also considered the more

gradual onset of oronasal breathing observed in adults and

children by the Bennett et al. [19,20] studies, herein termed

as “gradual augmenters.”

The gradual augmenter breathing habit for children was

estimated by linear regression of the observed minute ventilation and Fm at rest and at 40% maximum physical work

capacity from data in Table two and Figure two of Bennett

et al. [19] for 12 children (9 M, 3 F; 6-10 yrs of age). The

gradual augmenter breathing habit for adult males and females was estimated by linear regression of the observed

minute ventilation and Fm at rest and at 60% maximum

physical work capacity from data in Table two and

Figure three of Bennett et al. [20] for 22 adults (11 M, 11

F; mean age, 22 yrs). In the adult females, the fitted Fm was

zero for the activity of sitting and so was also set to zero

for the activity of sleep. Table 3 provides the Fm for all

breathing habits (normal augmenters, mouth-breathers,

and gradual augmenters) used in our simulations. In a

study of 37 subjects from 7-72 years of age, James et al.

[21] reported that 2 subjects (5.4%) breathed orally only.

With this finding in mind, we have also considered purely

oral breathing in our estimates of particle penetration into

regions of the lower respiratory tract.

For air passing through the mouth, deposition of large

particles by impaction occurs mainly at the larynx. From

Eq D.30 of ICRP [12], laryngeal deposition efficiency,

η(ET)larynx, is given by:

ηð Þ ET larynx 1⁄4 1− 1:1  10−4 d2

a Qtotal SF3  0:6 Vt SFt

3  −0:2 h i 1:4

þ 1

 −1

ð1Þ

where: da is aerodynamic diameter (μm); Qtotal is total

inspiratory flow rate (mL/s); VT is tidal volume (mL); and

SFt is a scaling factor of 1.0 for adult males, 1.08 for adult

females, and 1.26 for ten year-old children from Table

fifteen of ICRP [12].

For nasal breathing, ET deposition efficiency due to

impaction was calculated from Eq. D.32 and D.33 of

ICRP [12]. The ET deposition efficiencies for the anterior, η(ET1)nose, and posterior, η(ET2)nose, nasal regions

are given by:

ηð Þ ET1 nose 1⁄4 0:5 1− 3  10−4 d2

a Qnose SFt

3   þ 1  −1 n o

ð2Þ

ηð Þ ET2 nose 1⁄4 1− 5:5  10−5 d2

a Qnose SFt

3  1:17 þ 1

h i−1

ð3Þ

where: Qnose is the inspiratory flow (mL/s) through the

nose. The use of SFt in Equations 2 and 3 presumes that

Table 2 Daily ventilation rates (Vdaily).a

Vdaily (m3/day)

Very sedentary Median Highly active

5th percentile 50th percentile 95th percentile

Male b 12.86 17.48 22.11

Female b 9.91 13.67 17.42

Children c 7.20 10.22 13.24

a Data are from Table 2 of Brochu et al. [16]; b values for age range of 23 to <30

years; c average of male and female values for age range of 7 to <11 years.

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nasal deposition efficiency increases with decreasing body

size and increasing nasal resistance. Two studies [19,22]

suggest that the nasal deposition in children is less than that

of adults. These two studies, not considered in the ICRP

model [12], suggest that it may be inappropriate to apply a

scaling factor for nasal deposition of children. Accordingly,

we estimated the nasal deposition efficiency of the 10 yr old

child for a SFt of both 1.0 (child-A) and 1.26 (child-B). Additionally, we estimated the upper and lower 95% confidence

bounds for inter-individual variability attributable to differences in deposition efficiency within the ET region predicted

by Equations 1-3 as specified in paragraphs D44 and D68 of

ICRP [12].

The deposition efficiencies along the ET pathways (i.e.,

nasal and oral) were assumed to be independent. As such,

total ET deposition was taken to be the sum of deposition

between the pathways weighted by the flow partitioning (see

Paragraph 161 of ICRP [12]). The thoracic fraction, defined

as particle penetration past the larynx, P(ET), is given by:

P ET ð Þ1⁄4 1− Fm ηð Þ ET larynx−ð Þ 1−Fm

 ηð Þ ET1 nose þ 1− ηð Þ ET1 nose  ηð Þ ET2 nose  

ð4Þ

We estimated inspiratory deposition efficiency in the

TB region, ηTB, of particles (0.5-20 μm; 0.1 μm increments) using the publicly available multiple path particle

dosimetry (MPPD; ver 2.1, © 2009) model.d The model

considers deposition by the mechanisms of impaction,

sedimentation, and diffusion. The approach and formula

used to calculate particle losses in the MPPD model are

described by Anjilvel and Asgharian [23]. Physiological

input parameters (namely, tidal volume [VT], breathing

frequency [f], functional residual capacity [FRC], and

upper respiratory tract volume [URT]), necessary for

MPPD simulations are provided in Tables 1 and 4. FRC

and URT for each group are from Table fifteen of ICRP

[12]. The Yeh and Schum [24] typical path whole lung

model was utilized and scaled for FRC and VT. The effects of these physiologic parameters on deposition in

humans free of respiratory disease are described by de

Winter-Sorkina and Cassee [25].

The respiratory fraction, defined as particle penetration through the ciliated airways of the TB region, P

(TB), is given by:

P TB ð Þ1⁄4 ∫ Tinh

0 C1 dt

∫ Tinh

0 C0 dt

≅P ET ð Þ 1−ηTB ðÞ ð5Þ

where: C0 and C1 are particle concentration passing the

larynx and terminal bronchioles, respectively; and Tinh is

the time of inhalation. Since conducting airway particle

concentration is nearly constant during inhalation, respiratory fraction can be expressed in terms of TB deposition

efficiency as given above. An Additional file 1: Appendix

to this paper provides estimates of P(TB) based on the

ICRP [12] model rather than the MPPD model.

After calculating P(ET) and P(TB) for all activities and

individual groups, daily average estimates of P(ET)avg

and P(TB)avg weighted by daily ventilation (see Tables 1

and 2) were calculated as a function of particle size.

Ventilation-weighted averages of P(ET)avg for each particle size were calculated as:

P ET ð Þavg 1⁄4 Xn

i1⁄41

P ET ð Þi Vdaily−i

 =

Xn

i1⁄41

Vdaily−i

ð6Þ

where: P(ET)i is the ET fraction for activity, i; Vdaily-i is the

daily volume inhaled while engaged in activity, i; and n is

the number of activities. Ventilation-weighted averages of

P(TB)avg were computed similarly to those of P(ET)avg.

Finally, thoracic and respiratory particle fractions were also

calculated after applying the ICRP [12] inhalability criterion

assuming no ambient wind, 1−0.5 [1− (0.00076 da

2.8 + 1) −1

].

The ICRP [12] criterion was utilized as it better represents

the inhalation of particles <10 μm than the ACGIH and

CEN [2,3] criterion.

Results

We estimated particle penetration fractions into the thorax

and respiratory region of an adult male, adult female, and a

Table 3 Partitioning of breaths through the mouth and

nose

Sleeping Sitting Light Heavy

Exercise Exercise

Adult

Male

Normal

Augmenter a

0, 1.00 d 0, 1.00 0, 1.00 0.52, 0.48

Mouthbreather a0.29, 0.71 0.36, 0.64 0.59, 0.41 0.66, 0.34

Gradual

Augmenter b

0.12, 0.88 0.13, 0.87 0.29, .071 0.54, 0.46

Adult

Female

Normal

Augmenter a

0, 1.00 0, 1.00 0, 1.00 0.50, 0.50

Mouthbreather a

0.12, 0.88 0.23, 0.77 0.57, 0.43 0.65, 0.35

Gradual

Augmenter b

0, 1.00 0, 1.00 0.22, 0.78 0.59, 0.41

Child Normal

Augmenter a

0, 1.00 0, 1.00 0, 1.00 0.51, 0.49

Mouthbreather a0.29, 0.71 0.36, 0.64 0.59, 0.41 0.66, 0.34

Gradual

Augmenter c

0.29, 0.71 0.31, 0.69 0.51, 0.49 0.77, 0.23

a From regression equations for oral breathing in Figure 1 of Niinimaa et al.

[17]; b Data based on Table 2 and Figure 3 of Bennett et al. [20] for 22

individuals (11 M, 11 F; mean age, 22 yrs); c Data based on Table 2 and

Figure 2 of Bennett et al. [19] for 12 children (9 M, 3 F; 6-10 yrs of age); d

fraction inhaled through mouth, fraction inhaled through nose.

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10 yr old child. In the results described below, data do not

consider particle inhalability unless specifically stated.

Inhalability was not considered since, as the results will

show, it has a minimal affect on 50% cut-points for particle

penetration into the lower airways for all breathing habits

except the less probable condition of oral breathing.

Route of breathing

Of the factors we considered, route of breathing (or breathing habit) had the greatest effect on estimates of P(ET) and

P(TB). Figure 2 and 3 illustrate P(ET) and P(TB) for an

adult male engaged in light exercise as a function of Fm. In

Figure 2, fifty percent P(ET) occurs at the da of 1.8 , 2.7,

4.4, 6.1, and 7.0 μm for the Fm of 0.00, 0.25, 0.50, 0.75, and

1.00, respectively. Note that the curve in Figure 2 for Fm=1

is for comparable conditions to those for which ACGIH [4]

thoracic fraction was based, i.e. an orally breathing adult

male engaged in light exercise. In Figure 3, fifty percent P

(TB) occurs at the da of 1.7 , 2.5, 3.8, 5.1, and 5.7 μm for

the Fm of 0.00, 0.25, 0.50, 0.75, and 1.00, respectively. As

ventilation is shifted to the lower removal efficiency oral

passages, there is an ever greater separation between the P

(ET) and P(TB) curves. By contrast, for purely nasal breathing (Fm = 0, the case for normal augmenters during light

exercise), due to the vast removal of particles in the nasal

airways, there is nearly no difference between the P(ET)

and P(TB) curves in Figures 2 and 3, respectively.

Table 5 provides the 50% cut-points for particle penetration into the thorax and respiratory region for all the

breathing habits we evaluated. As may be expected based

on Fm (see Table 3), the predicted particle penetration for

the gradual augmenter breathing habit is enveloped between that of the normal augmenter and mouth-breathers.

Additionally, consistent with Figures 2 and 3, Table 5

shows that the largest 50% cut-points are observed during

the case of oral breathing. Table 6 provides data on the

penetration of 10 μm particles into the lower respiratory

tract which is generally less than 20%, except for the case

of oral breathing where penetration into the thorax can

approach 40%.

Age and sex

Daily weighted penetrations curves for P(ET)avg and

P(TB)avg are illustrated in Figures 4 and 5, respectively.

For normal augmenters and oral breathing, 50% cutpoints for P(ET)avg and P(TB)avg were generally similar

between adult males and females, but shifted to slightly

(<0.2 μm) smaller particle sizes in the females (see Table 5).

There was a larger (<0.8 μm) difference in 50% cut-points

between males and females for the mouth-breather and

gradual augmenter breathing habits which is attributable

to greater nasal inhalation by females than males.

The data for child-A are more consistent, than child-B,

with experimental data [19,22] showing a lower nasal particle removal efficiency in children than adults. For all

breathing habits except oral breathing in Table 5, child-A

shows larger 50% cut-points than adults. Additionally, in

Table 6, the penetration of 10 μm particles into the thoracic and respiratory regions of child-A is generally greater

than or equal to that of adults for all breathing habits other

Table 4 Functional residual volume (FRC) and upper

respiratory tract volumes (URT)

FRC (mL) URT (mL)

Adult Male 3300 50

Adult Female 2680 40

Child (10 yrs) 1484 25

0.0

0.2

0.4

0.6

0.8

1.0

0 2 4 6 8 10

P(ET)

da (μm)

Figure 2 Thoracic fraction, i.e., particle penetration through the

extrathoracic region, P(ET), as a function of breathing route.

Penetration data are with respect to particle diameter as a function

of the fraction of air inhaled through the mouth (Fm) in an adult

male engaged in light exercise relative to particles entering the

respiratory tract. Curves are for the Fm of 0.00, 0.25, 0.50, 0.75, and

1.00 as indicated on the figure. Horizontal red line highlights

50% penetration.

0.0

0.2

0.4

0.6

0.8

1.0

0 2 4 6 8 10

P(TB)

da (μm)

Figure 3 Respirable fraction, i.e., particle penetration through

the tracheobronchial region, P(TB), as a function of breathing

route. Penetration data are with respect to particle diameter as a

function of the fraction of air inhaled through the mouth (Fm) in an

adult male engaged in light exercise relative to particles entering

the respiratory tract. Curves are for the Fm of 0.00, 0.25, 0.50, 0.75,

and 1.00 as indicated on the figure. Horizontal red line highlights

50% penetration.

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than oral. For oral breathing, the penetration of 10 μm particles is lower in child-A than adults.

Activity level

Impaction in the nasal airways, larynx, and large bronchi increases in conjunction with activity and increasing inspiratory flows across the range of da. Therefore, decreasing 50%

cut-points for both P(ET)avg and P(TB)avg are observed with

increasing activity. In general, the penetration of 10 μm particles into the thoracic and respiratory regions also showed

a small reduction with increasing daily activity level (see

Table 6). However, the small reductions in particle

penetration pale in comparison to the large increases in

ventilation and intake of particles that occur with increasing

activity level.

Inhalability adjustment

The thoracic and respirable fraction data in Table 5 were

relative to particles entering the respiratory tract. For comparison, Table 7 provides da associated with 50% penetration into the thorax and respiration regions after applying

inhalability criterion. Adjusting for inhalability shifts penetration curves to smaller particle sizes, and this effect is

most evident where there is a low activity level and a

Table 5 Particle penetration (50% cut-point) through respiratory tract regions relative to particles entering the

respiratory tract

Vdaily (%-tile) Normal Gradual

Augmenter Mouth-breather Augmenter Oral only

P(ET)avg P(TB)avg P(ET)avg P(TB)avg P(ET)avg P(TB)avg P(ET)avg P(TB)avg

Male

5% 2.94 a 2.74 5.15 4.32 3.60 3.25 9.00 6.67

95% CI (1.72–5.03) (1.65–4.34) (3.12–8.48) (2.84–6.10) (2.11–6.11) (2.00–4.98) (5.86–13.8) (5.00–8.14)

50% 2.46 2.30 5.09 4.30 3.37 3.05 8.09 6.11

95% CI (1.44–4.20) (1.38–3.71) (3.14–8.26) (2.86–5.88) (1.99–5.67) (1.88–4.65) (5.26–12.4) (4.58–7.34)

95% 2.14 2.03 5.08 4.31 3.19 2.90 7.61 5.89

95% CI (1.25–3.66) (1.20–3.32) (3.16–8.16) (2.89–5.83) (1.90–5.33) (1.79–4.44) (4.95–11.7) (4.38–7.07)

Female

5% 2.92 2.71 4.32 3.78 3.10 2.86 8.78 6.50

95% CI (1.71–5.01) (1.63–4.28) (2.57–7.22) (2.40–5.52) (1.81–5.30) (1.73–4.48) (5.71–13.5) (4.88–7.91)

50% 2.42 2.27 4.46 3.87 2.91 2.68 7.88 5.96

95% CI (1.41–4.13) (1.36–3.64) (2.72–7.32) (2.51–5.45) (1.71–4.94) (1.63–4.19) (5.13–12.1) (4.47–7.17)

95% 2.10 1.99 4.58 3.96 2.74 2.54 7.44 5.77

95% CI (1.23–3.60) (1.18–3.26) (2.82–7.42) (2.60–5.49) (1.61–4.64) (1.53–4.00) (4.84–11.4) (4.29–6.94)

Child-B b

5% 2.77 2.61 4.31 3.78 4.30 3.78 8.34 6.46

95% CI (1.62–4.75) (1.56–4.18) (2.58–7.19) (2.40–5.57) (2.57–7.17) (2.40–5.56) (5.42–12.8) (4.74–8.04)

50% 2.25 2.13 4.37 3.81 4.07 3.59 7.50 5.81

95% CI (1.31–3.85) (1.27–3.48) (2.67–7.15) (2.47–5.35) (2.46–6.71) (2.29–5.15) (4.88–11.5) (4.32–7.08)

95% 1.89 1.81 4.34 3.76 3.98 3.49 6.91 5.33

95% CI (1.11–3.24) (1.07–2.98) (2.69–7.03) (2.48–5.11) (2.45–6.47) (2.27–4.88) (4.49–10.6) (4.01–6.36)

Child-A c

5% 3.92 3.56 5.60 4.72 5.59 4.72 8.34 6.46

95% CI (2.29–6.72) (2.18–5.44) (3.38–9.28) (3.09–6.63) (3.37–9.27) (3.09–6.62) (5.42–12.8) (4.74–8.04)

50% 3.18 2.94 5.37 4.53 5.12 4.36 7.50 5.81

95% CI (1.86–5.45) (1.77–4.58) (3.29–8.77) (3.03–6.10) (3.11–8.40) (2.88–5.95) (4.88–11.5) (4.32–7.08)

95% 2.68 2.50 5.16 4.34 4.85 4.14 6.91 5.33

95% CI (1.56–4.58) (1.50–3.97) (3.19–8.34) (2.94–5.66) (2.98–7.90) (2.76–5.51) (4.49–10.6) (4.01–6.36)

P(ET)avg, extrathoracic particle penetration which is the thoracic particle fraction averaged across all activity levels weighted by daily ventilation; P(TB)avg,

tracheobronchial particle penetration which is the respirable particle fraction averaged across all activity levels weighted by daily ventilation; 95% CI, ninety-five

percent confidence intervals for inter-individual variability attributable to differences in particle penetration through the extrathoracic region; a Aerodynamic

particle diameter in μm; b Scaling factor in Equations 2 and 3 equal to 1.26; c Scaling factor in Equations 2 and 3 equal to 1.0.

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substantial contribution of oral breathing. Table 6 provides

data on the penetration of 10 μm particles into the lower

respiratory tract. Those penetration data may be adjusted

for inhalability by multiplying by 0.84, the inhalability of

10 μm particles.

Discussion

We calculated thoracic and respirable particle fractions

for an adult male, adult female, and ten year-old child

engaged in typical daily activities ranging from sleep to

heavy exercise. Our estimates are intended to represent

full-day ambient and/or non-ambient exposures while

individuals are engaged in a variety of activities. This differs from the ACGIH and CEN criteria which are

intended to represent a workplace setting [2,4]. Similarly,

considering the need to provide protection for sensitive individuals who may breathe by mouth and/or oronasally, the

EPA [8] selected the nominal cut-point of 10 μm as an

Table 6 Penetration of 10 μm (da) through respiratory tract regions relative to particles entering the respiratory tract

Vdaily

(%-tile)

Normal Gradual

Augmentera Mouth-breathera Augmentera Oral onlya

P(ET)avg P(TB)avg P(ET)avg P(TB)avg P(ET)avg P(TB)avg P(ET)avg P(TB)avg

Male

5% 0.05b 0.03 0.21 0.10 0.11 0.05 0.43 0.21

50% 0.04 0.02 0.19 0.08 0.11 0.04 0.36 0.15

95% 0.03 0.01 0.19 0.06 0.10 0.03 0.32 0.12

Female

5% 0.06 0.03 0.15 0.07 0.06 0.03 0.41 0.20

50% 0.04 0.02 0.15 0.06 0.07 0.02 0.35 0.14

95% 0.03 0.01 0.16 0.05 0.07 0.02 0.31 0.11

Child-B c

5% 0.05 0.02 0.16 0.08 0.16 0.08 0.38 0.18

50% 0.03 0.01 0.15 0.06 0.14 0.06 0.31 0.13

95% 0.02 0.01 0.14 0.04 0.13 0.04 0.27 0.09

Child-A d

5% 0.10 0.05 0.21 0.10 0.21 0.10 0.38 0.18

50% 0.07 0.04 0.19 0.09 0.18 0.07 0.31 0.13

95% 0.05 0.02 0.17 0.05 0.16 0.05 0.27 0.09

P(ET), extrathoracic penetration for 10 μm particles; P(TB), tracheobronchial penetration for 10 μm particles; aData are daily averages, all activity levels weighted by

daily ventilation; b

Fraction penetration; cScaling factor in Equations 2 and 3 equal to 1.26; dScaling factor in Equations 2 and 3 equal to 1.0.

0.0

0.2

0.4

0.6

0.8

1.0

0 2 4 6 8 10

P(ET)avg

da (μm)

10 yr-old Child-A

10 yr-old Child-B

Adult Male

Adult Female

Figure 4 Thoracic fraction, i.e., particle penetration through the

extrathoracic region, P(ET)avg, in adults and a 10 yr-old child.

Data are daily averaged values for a median activity level, gradual

augmenter breathing habit, and uncorrected for particle inhalability.

Child-A and child-B are for a scaling factor of 1.0 and 1.26 in

Equations 2 and 3, respectively. Horizontal red line highlights 50%

penetration which occurs at 3.1 μm (adult female), 3.4 μm (adult

male), 4.1 μm (child-B), and 5.1 μm (child-A).

0.0

0.2

0.4

0.6

0.8

1.0

0246 8 10

P(TB)avg

da (μm)

10 yr-old Child-A

10 yr-old Child-B

Adult Male

Adult Female

Figure 5 Respirable fraction, i.e., particle penetration through

the tracheobronchial region, P(TB)avg, in adults and a 10 yr-old

child. Data are daily averaged values for a median activity level,

gradual augmenter breathing habit, and uncorrected for particle

inhalability. Child-A and child-B are for a scaling factor of 1.0 and

1.26 in Equations 2 and 3, respectively. Horizontal red line highlights

50% penetration which occurs at 2.7 μm (adult female), 3.1 μm

(adult male), 3.6 μm (child-B), and 4.4 μm (child-A).

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indicator of the thoracic fraction consistent with ISO

[26,27] recommendations for occupational or nonoccupational environments. Our estimates show less penetration of coarse particulate matter into the thoracic and

gas exchange regions of the respiratory tract than current

criteria. For typical breathing habits (i.e., not oral breathing), we would predict less than 20% penetration of 10 μm

particles into the thorax, whereas a 50% penetration of 10

μm is currently used in both occupational and nonoccupational criteria [2,4,8,9]. Recognizing that there are

differences in the sources and chemical composition between ambient fine (nominal mean da ≤ 2.5 μm) and larger

coarse PM, our finding may, in part, explain why causal

relationships are observed between morbidity and mortality

with short and long-term exposure to fine PM but not larger coarse PM (see Chapter 2 in Ref [28]).

There are two primary reasons for the dramatic difference between our estimates and the current criteria. First,

the ACGIH [4] criteria considered all inspired air to enter

via the oral airway which increases the penetration

through the ET airways. With the exception of a laboratory setting, however, few individuals breathe exclusively

through the mouth. This would make the breathing habits

other than oral breathing preferable for the purposes of

estimating actual exposures. Second, the ACGIH criteria

are intentionally conservative (Figure 1) as the committee

Table 7 Inhalability adjusted particle penetration (50% cut-point) through respiratory tract regions

Vdaily

(%-tile)

Normal Gradual

Augmenter Mouth-breather Augmenter Oral only

P(ET)avg P(TB)avg P(ET)avg P(TB)avg P(ET)avg P(TB)avg P(ET)avg P(TB)avg

Male

5% 2.92 a 2.72 4.97 4.24 3.55 3.22 8.24 6.38

95% CI (1.71–4.90) (1.64–4.26) (3.09–7.66) (2.81–5.85) (2.11–5.82) (2.00–4.85) (5.66–11.4) (4.89–7.59)

50% 2.44 2.30 4.93 4.21 3.33 3.02 7.52 5.90

95% CI (1.43–4.12) (1.37–3.67) (3.11–7.51) (2.84–5.67) (1.99–5.43) (1.87–4.54) (5.12–10.5) (4.50–6.98)

95% 2.14 2.02 4.92 4.22 3.15 2.88 7.14 5.71

95% CI (1.25–3.62) (1.20–3.29) (3.13–7.44) (2.87–5.64) (1.89–5.15) (1.78–4.36) (4.84–10.1) (4.32–6.77)

Female

5% 2.90 2.70 4.22 3.72 3.08 2.85 8.06 6.23

95% CI (1.70–4.87) (1.63–4.20) (2.56–6.73) (2.38–5.34) (1.81–5.13) (1.73–4.40) (5.53–11.2) (4.78–7.41)

50% 2.41 2.26 4.36 3.81 2.89 2.67 7.35 5.77

95% CI (1.41–4.06) (1.35–3.60) (2.70–6.80) (2.50–5.29) (1.71–4.79) (1.63–4.12) (5.00–10.3) (4.39–6.83)

95% 2.10 1.99 4.46 3.89 2.72 2.52 7.00 5.60

95% CI (1.23–3.55) (1.18–3.23) (2.80–6.87) (2.58–5.33) (1.61–4.42) (1.53–3.93) (4.74–9.91) (4.22–6.66)

Child-B b

5% 2.76 2.60 4.22 3.73 4.21 3.72 7.73 6.20

95% CI (1.62–4.64) (1.56–4.11) (2.56–6.69) (2.39–5.38) (2.56–6.68) (2.39–5.37) (5.28–10.8) (4.65–7.52)

50% 2.24 2.13 4.27 3.75 3.99 3.54 7.05 5.63

95% CI (1.31–3.79) (1.27–3.44) (2.65–6.66) (2.46–5.20) (2.45–6.30) (2.28–5.01) (4.77–9.98) (4.25–6.74)

95% 1.89 1.80 4.25 3.71 3.91 3.45 6.56 5.21

95% CI (1.10–3.21) (1.07–2.96) (2.67–6.56) (2.47–4.99) (2.42–6.12) (2.25–4.78) (4.42–9.37) (3.96–6.15)

Child-A c

5% 3.87 3.52 5.40 4.62 5.39 4.61 7.73 6.20

95% CI (2.28–6.36) (2.17–5.27) (3.34–8.32) (3.07–6.32) (3.34–8.31) (3.06–6.31) (5.28–10.8) (4.65–7.52)

50% 3.15 2.92 5.20 4.44 4.96 4.28 7.05 5.63

95% CI (1.85–5.29) (1.77–4.49) (3.26–7.96) (3.00–5.88) (3.09–7.66) (2.86–5.75) (4.77–9.98) (4.25–6.74)

95% 2.66 2.49 5.00 4.26 4.73 4.07 6.56 5.21

95% CI (1.56–4.47) (1.50–3.91) (3.16–7.64) (2.92–5.51) (2.96–7.29) (2.74–5.39) (4.42–9.37) (3.96–6.15)

P(ET)avg, extrathoracic particle penetration which is the thoracic particle fraction averaged across all activity levels weighted by daily ventilation; P(TB)avg,

tracheobronchial particle penetration which is the respirable particle fraction averaged across all activity levels weighted by daily ventilation; 95% CI, ninety-five

percent confidence intervals for inter-individual variability attributable to differences in particle penetration through the extrathoracic region; a Aerodynamic

particle diameter in μm; b Scaling factor in Equations 2 and 3 equal to 1.26; c Scaling factor in Equations 2 and 3 equal to 1.0.

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chose to afford extra protection by over representing the

true penetration of particles into the lower respiratory

tract. In Figure 2, we predicted a 50% cut-point of 7.0 μm

for the conditions considered by the ACGIH, namely, an

orally breathing adult male engaged in light exercise. Additionally, our predicted upper bound 95th percentile for

50% cut-points during oral breathing corrected for

inhalability in Table 7 are ~10 μm. Thus, selection of 10

μm as having 50% penetration into the thorax was consistent with over representing the true penetration of particles

into the lower respiratory tract of most individuals.

Route of breathing has a dramatic affect on particle delivery to the thoracic and respiratory regions since the deposition efficiency of the nasal passages greatly exceeds that of

the oral pathway. Most subjects in the Niinimaa et al. [17]

study, 87% (26 of 30), breathed through their nose until an

activity level was reached when they switched to oronasal

breathing. Thirteen percent (4 of 30) of the subjects, however, were oronasal breathers even at rest. These two subject groups are commonly referred to in the literature (e.g.,

see [12]) as “normal augmenters” and “mouth-breathers,”

respectively. Becquemin et al. [18] and Bennett et al. [19]

showed that children tend to have a greater fraction of oral

breathing than adults at rest and during exercise. Route of

breathing may also vary between races; Bennett et al. [20]

found that African-Americans and females had a greater

nasal contribution to breathing during exercise than Caucasians and males. The abrupt change in route of breathing

occurring in normal augmenters has not been observed by

others. The gradual augmenter breathing habit based on

Bennett et al. [19,20] may be preferable to the normal augmenter in representing the general population. Chadha

et al. [29] found that the majority (11 of 12) of patients with

asthma or allergic rhinitis also breathe oronasally at rest. In

healthy individuals, a small fraction (around 5%) may

breathe solely through the mouth [21]. Our estimates for

gradual augmenters provide particle penetration fractions

most typical of healthy populations. Our estimates for

mouth-breathers may be more appropriate for patients

with mild upper respiratory disease.

The ICRP model [12] appears to underestimate the

penetration of particles through the ET airways of children.

A SFt is applied in Equations 1-3 with the presumption

that oral and nasal particle deposition increase with decreasing body size and increasing flow resistance.

For oral breathing on a mouthpiece, Bennett et al. [30]

showed greater ET deposition in children than adults. This

finding suggests that the application of a SFt of 1.26 in

Eq 1 is appropriate for laryngeal deposition. However, for

nasal breathing, Becquemin et al. [22] and Bennett et al.

[19] showed less nasal deposition in children than in

adults. These two studies, not considered in the ICRP

model [12], suggest that it may be inappropriate to apply a

SFt in Equations 2 and 3 for nasal deposition in children.

Lower nasal deposition of particles in children than adults

means greater penetration of particles into the lower

respiratory tract of children than adults. Accordingly,

we conducted simulations for the child with the SFt in

Equations 2 and 3 set equal to 1.0 in addition to the SFt of

1.26 recommended by ICRP [12]. The estimated nasal ET

deposition efficiency of 2 μm particles in the normal augmenter child during light exercise decreased from 68% to

48% when the SFt was decreased from 1.26 to 1.0. For

comparison, under the same level of activity, the estimated

ET deposition efficiency was 57-58% in the adult male and

female. Decreasing the nasal deposition efficiency of the

child relative to the ICRP model [12] increased the particle

size estimated to have 50% penetration into the thoracic

and respiratory regions (see Tables 5, 6, 7 and Figures 4

and 5). These estimates of larger 50% cut points for childA than adults appear consistent with studies in children

that were not incorporated into the ICRP model [12].

With the exception of Table 7, the thoracic and respirable fractions that we present are the amount of particles

entering a specified respiratory tract region relative to the

amount of particles entering the respiratory tract. In effect,

we assumed 100% inhalability across the range of particle

sizes (0.5-20 μm) examined. We have opted on this convention since the inhalable fraction depends on factors not

considered here such as wind speed and direction relative

to the exposed individual. For recent reviews of the literature on particle inhalability, the reader is referred to

Brown [31] and Millage et al. [32]. Adjusting our data for

inhalability, shifts penetration curves to smaller particle

sizes, but mainly only where there is a substantial contribution of oral breathing (see Table 7 vs. Table 5).

Conclusion

Our analyses show that occupation and non-occupational

criteria for thoracic and respirable fractions overestimate

the size of particles entering these regions. As already

noted, penetration fractions for workplace criteria were

chosen to afford extra protection by over-representing the

true penetration of particles into regions of the respiratory

tract [4]. However, accepted definitions for thoracic and

respirable fractions speak specifically to particles that penetrate into these regions. As such, current occupational and

non-occupational criteria may misinform practitioners

with regard to the actual size of particles expected to reach

regions of the respiratory tract during typical behavior. For

instance, the current criteria suggest that 10 μm particles

(50%) penetrate into the thorax, thus, leaving the expectation that observed health effects may be modulated by

their deposition in either the upper or lower airways. However, we predict that about 20% or less of these 10 μm particles would penetrate through the ET airways and into the

lower respiratory tract. Our modifications to the ICRP

model [12] related to breathing habit and nasal deposition

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in children reflect more recent data and provide consistent

estimates of greater particle penetration into the thoracic

and respiratory regions of children than adults. With those

modifications, for median activities, we predict 50% cutpoints for P(ET)avg at ~3 μm in adults and ~5 μm in children. The predicted 50% cut-points for P(TB)avg are slightly

less than 3 μm in adults and slight greater than 4 μm in

children. Our estimates of particle penetration into the

thoracic and respiratory regions of the respiratory tract

should be useful in the design of experimental studies and

interpretation of PM health effects evidence.

Endnotes a

More typically, the literature has defined this term in relation to the fraction of particles entering the gas-exchange

region or the fraction penetrating through the tracheobronchial region, the ciliated airways, or conducting airways.

b

For completeness, other groups such as the British

Medical Research Council offered size-selective sampling

recommendations prior to the ACGIH. For a historical

perspective, the reader is referred to Lippmann [33]. c

For accuracy it should be recognized that the sampler

collection efficiency curves for EPA’s PM10 and ACGIH’s

thoracic fraction are different. The criteria are similar for

particles smaller than the 50% cut-point at 10 μm. However, the curves diverge at about 12 μm, with a dramatic

drop in collection efficiency (dictated by policy considerations) for EPA’s PM10, and a more gradual decrease in

collection efficiency for the ACGIH criterion.

d

The MPPD model typically outputs estimates of regional

deposition for the entire respiratory cycle. For the purposes

of this project, the software output was modified by the developers to provide inspiratory deposition fractions for particles in the ET and TB regions. Designating the ET and TB

regions as separate compartments, the deposition efficiency

in the TB region (ηTB) during inhalation was calculated

from the MPPD output as DFTB / (1-DFET), where DFTB

and DFET are the deposition fractions of particles in the TB

and ET region during inhalation, respectively. For more information about this model, the reader is referred to: http://

www.ara.com/products/mppd_capabilities.htm.

Additional file

Additional file 1: Comparison of respiratory particle fractions

predicted by the MPPD and ICRP [12] models. In general, the ICRP

[12] model predicts less particle penetration into the respiratory region

than the MPPD model.

Abbreviations

ACGIH: American Conference of Governmental Industrial Hygienists;

CEN: European Committee for Standardization; da: Aerodynamic diameter;

DFTB: Particle deposition fraction in the TB region during inhalation;

DFET: Particle deposition fractions in the ET region during inhalation; EPA: U.

S. Environmental Protection Agency; ET: Extrathoracic; f: Breathing frequency;

Fm: Fraction of breath passing through the mouth; FRC: Functional residual

capacity; ICRP: International Commission on Radiological ProtectionISO

International Organization for Standardization; MPPD: Multiple path particle

dosimetry; NAAQS: National ambient air quality standard; ηTB: Particle

deposition efficiency in the tracheobronchial region; η(ET)larynx: Extrathoracic

particle deposition efficiency in the larynx; η(ET1)nose: Extrathoracic particle

deposition efficiency in anterior nasal region; η(ET2)nose: Extrathoracic particle

deposition efficiency in posterior nasal region; P(ET): Particle penetration past

the larynx and the thoracic fraction; P(TB): Particle penetration through the

ciliated airways and the respirable fraction; PM: Particulate matter;

PM2.5: Particles with a nominal mean aerodynamic diameter ≤ 2.5 μm;

PM10: Indicator for thoracic coarse particles; Qnose: Inspiratory flow through

the nose; Qtotal: Total inspiratory flow rate; SFt: Scaling factor, ratio of trachea

diameter in adult reference male to that of subject; t: Time spent engaged in

specific activity; TB: Tracheobronchial; TSP: Total suspended particulate;

URT: Upper respiratory tract volume; Vdaily: Total volume inspired in 24 hours;

VT: Tidal volume.

Competing interests

The authors have no competing interest.

Authors’ contributions

JB conceived the project, coordinated and drafted the manuscript. TG

contributed to defining the project’s scope and drafting the manuscript. OP

modified software for the purposes of this project to provide inspiratory

deposition fractions for particles and participated in the interpretation of

data. BA contributed to the methodology, participated in software

development and drafting of the manuscript. All authors read and approved

the final manuscript.

Acknowledgments

The authors thank Drs. Beverly Cohen (NYU), Martin Harper (NIOSH), Mort

Lippmann (NYU), and Lindsay Wichers Stanek (U.S. EPA) for their helpful

comments. TG was funded, in part, by a faculty appointment with the U.S.

EPA through a program administered by Oak Ridge Institute for Science and

Education (EPA-ORD/NCEA-RTP-2009-02). This document has been reviewed

in accordance with U.S. Environmental Protection Agency policy and

approved for publication. Mention of trade names or commercial products

does not constitute endorsement or recommendation for use. The views

expressed in this article are those of the authors and do not necessarily

reflect the views or policies of the U.S. Environmental Protection Agency.

Author details

1

National Center for Environmental Assessment, U.S. Environmental

Protection Agency, MD B243-01, Research Triangle Park, Raleigh, NC 27711,

USA. 2

NYU School of Medicine, 57 Old Forge Road, Tuxedo, NY 10987, USA. 3

Applied Research Associates, Inc, 801 N. Quincy St., Suite 700, Arlington, VA

22203, USA. 4

Applied Research Associates, Inc, 8537 Six Forks Road., Suite

600, Raleigh, NC 27615, USA.

Received: 4 May 2012 Accepted: 3 February 2013

Published: 10 April 2013

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doi:10.1186/1743-8977-10-12

Cite this article as: Brown et al.: Thoracic and respirable particle

definitions for human health risk assessment. Particle and Fibre Toxicology

2013 10:12.

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