1. Typical Materials Composition of a Tire

This table lists the typical types of materials used to manufacture tires.

Typical Composition of a Tire

Synthetic RubberNatural Rubber

Sulfur and sulfur compounds Silica

Phenolic resin Oil: aromatic, naphthenic, paraffinic

Fabric: Polyester, Nylon, Etc. Petroleum waxes

Pigments: zinc oxide, titanium dioxide, etc. Carbon black Fatty acids

Inert materials Steel Wire

 

2. Typical Composition by Weight

This lists the major classes of materials used to manufacture tires by the percentage of the total weight of the finished tire that each material class represents.

Passenger Tire

Natural rubber 14 %Synthetic rubber 27%Carbon black 28%Steel 14 - 15%Fabric, fillers, accelerators, antiozonants, etc.

16 - 17%

Average weight: New 25 lbs, Scrap 20 lbs.

 

Truck Tire

Natural rubber 27 %Synthetic rubber 14%Carbon black 28%Steel 14 - 15%Fabric, fillers, accelerators, antiozonants, etc.

16 - 17%

Average weight: New 120 lbs., Scrap 100 lbs.

3. Densities of Shredded and Whole Tires

APPROXIMATE DENSITIES

LOOSELY PACKED   DENSELY PACKED550-600 lbs/yd3 single pass 1220-1,300 lbs/yd3

850-950 lbs/yd3 2" shred 1,350-1,450 lbs/yd3

1,000-1,100 lbs/yd3 1 1/2" shred 1,500-1,600 lbs/yd3

100/10Yd3 WHOLE TIRES(PASSENGER/LIGHT TRUCK)

500/10Yd3

  

10 MESH- 29 lbs/ft3

20 MESH- 28 lbs/ft3

30 MESH- 28 lbs/ft3

40 MESH- 27 lbs/ft3

80 MESH- 25-26 lbs/ft3

4. Rubber weight by tire component.

A tire is manufactured from several separate components, such as tread, innerliner, beads, belts, etc. This table shows which components account for the rubber used to make the tire.

RUBBER PERCENT BY WEIGHT IN A NEW RADIAL PASSENGER TIRE

 TREAD 32.6%BASE 1.7%SIDEWALL 21.9%BEAD APEX 5.0%BEAD INSULATION 1.2%FABRIC INSULATION 11.8%INSULATION OF STEEL CORD 9.5%INNERLINER 12.4%UNDERCUSHION             3.9%   100.0%

5. Steel Tire Cord Analysis

The tire industry uses ASTM 1070 and above tire cord quality wire rod in the manufacture of new tires. There are approximately 2.5 pounds of steel belts and bead wire in a passenger car tire.

The meaning of tire codes

Tire identification diagram

Tire identification diagram, light truck specific features

The ISO Metric tire code consists of a string of letters and numbers, as follows:

An optional letter (or letters) indicating the intended use or vehicle class for the tire:

o P: Passenger o LT: Light Truck o ST: Special Trailer o T: Temporary

3 digit number: The width of the tire in millimeters, from sidewall edge to sidewall edge.

/: Slash character 2 digit number: The ratio of the sidewall height to

the total width of the tire as a percentage. If this is missing, it is assumed to be 82%. If the number is larger than 200, then this is the diameter of the entire tire in millimeters.

Construction of the fabric of the tire: o B: bias belt o D: diagonal o R: radial

2 digit number: Diameter in inches of the rim that this tire is designed to fit.

2 or 3 digit number: Load index; see table below. 1 or 2 digit/letter combo: Speed rating; see table

below. Additional marks: See subheading below.

Some light truck tires follow the Light Truck Numeric or Light Truck High Flotation systems, indicated by the letters LT at the end instead of the beginning of the sequence, as follows:

The tire diameter is given for High Flotation tires and omitted from Numeric tires.

o 2 digit number: The diameter of the tire in inches.

o x: Separator character. 3 or 4 digit number: The section width (cross

section) of the tire in inches. If the tire diameter is not given, section widths ending in zero (e.g., 7.00 or 10.50) indicate the aspect ratio is 92%, while section widths not ending in zero (e.g., 7.05 or 10.55) indicate the aspect ratio is 82%.

Construction of the fabric of the tire: o B: bias belt o D: diagonal o R: radial

2 digit number: Diameter in inches of the rim that this tire is designed to fit.

LT: Designates that this is a Light Truck tire. Load index and speed rating are not required for

this type of tire but may be provided by the manufacturer.

o 2 or 3 digit number: Load index; see table below.

o 1 or 2 digit/letter combo: Speed rating; see table below.

Additional marks: See subheading below.

[edit] Load Index

The load index on a tire is a numerical code associated with the maximum load each tire can carry (i.e.: if the car is in perfect balance, each tire will hold one-forth the cars weight, however, cars are rarely ever in perfect balance).

Load Index

Code Pounds Kilograms Code Pounds Kilograms

71 761 345 91 1,356 615

72 783 355 92 1,389 630

73 805 365 93 1,433 650

74 827 375 94 1,477 670

75 853 387 95 1,521 690

76 882 400 96 1,565 710

77 908 412 97 1,609 730

78 937 425 98 1,653 750

79 963 437 99 1,709 775

80 992 450 100 1,764 800

81 1,019 462 101 1,819 825

82 1,047 475 102 1,874 850

83 1,074 487 103 1,929 875

84 1,102 500 104 1,984 900

85 1,135 515 105 2,039 925

86 1,168 530 106 2,094 950

87 1,201 545 107 2,149 975

88 1,235 560 108 2,205 1,000

89 1,279 580 109 2,271 1,030

90 1,323 600 110 2,337 1,060

[edit] Speed rating codes

The code is made up of one or two letters, or one letter and one number. It indicates the maximum speed that the tire can sustain for a ten minute endurance without being a danger.

Tire speed rating codes

Code mph km/h Code mph km/h

A1 3 5 L 75 120

A2 6 10 M 81 130

A3 9 15 N 87 140

A4 12 20 P 94 150

A5 16 25 Q 100 160

A6 19 30 R 106 170

A7 22 35 S 112 180

A8 25 40 T 118 190

B 31 50 U 124 200

C 37 60 H 130 210

D 40 65 V 149 240

E 43 70 Z over 149 over 240

F 50 80 W 168 270

G 56 90 Y 186 300

J 62 100 (Y) over 186 over 300

K 68 110

Prior to 1991, tire speed ratings were shown inside the tire size, replacing the "R" character. The available codes were SR (112 mph, 180km/h), HR (130 mph, 210 km/h), VR (in excess of 130 mph, 210 km/h) and ZR (in excess of 149 mph, 270 km/h). The ZR code is still in common use, often combined with a W or Y speed code.

Tires with a speed rating higher than 186 MPH (300 km/h) are indicated by a Y in parenthesis. The load rating is often included within the parenthesis, e.g. (86Y).

In some countries, tires are required by law to exceed the maximum speed of the vehicle they are mounted on with regards to their speed rating code. In Germany, tires that are not fit for a car's or motorcycle's particular maximum speed are illegal to mount unless a warning sticker stating the allowed maximum speed is placed within clear sight of the driver inside the vehicle. Some manufacturers will install a speed governor if a vehicle is ordered with tires rated below the vehicle's maximum speed.

[edit] Additional marks

There are numerous other markings on a typical tire, these may include:

M+S: Mud and Snow; winter/all-weather tires with above-average traction in muddy or snowy conditions. Spike tires have an additional letter, "E" (M+SE).

BSW: Black SideWall WSW: White SideWall

E4: Tire approved according ECE-regulations, the number indicating the country of approval.

030908: Approval number of the tire DOT code: All tires imported into the USA have

the DOT code, as required by the Department of Transportation (DOT). It specifies the company, factory, mold, batch, and date of production (2 digits for week of the year plus 2 digits for year; or 2 digits for week of the year plus 1 digit for year for tires made prior to 2000)

TL: Tubeless TT: Tube-type, tire with an inner-tube Made in: Country of production C: Commercial; tires for light trucks (Example: 185

R14 C) B: Bias belted; tires for motorcycles (Example:

150/70 B 17 69 H)—diagonal construction with belt added under the tread

SFI: Side Facing Inwards; inside of asymmetric tires

SFO: Side Facing Outwards; outside of asymmetric tires

TWI: Tread Wear Indicator; a device in the main tire profile which shows when the tire is worn down and needs to be replaced

SL: Standard Load; tire for normal usage and loads XL: eXtra Load; tire for heavy loads RF: Reinforced tires Arrows: Some tread designs are designed to

perform better when driven in a specific direction (clockwise or counter-clockwise). Such tires will have an arrow showing which way the tire should rotate when the vehicle is moving forwards. It is important not to put a "clockwise" tire on the left hand side of the car or a "counter-clockwise" tire on the right side.

To facilitate proper balancing, most high performance tyre manufacturers in addition place red (uniformity) and yellow (weight) marks on the sidewalls of its tyres to enable the best possible match-mounting of the tyre/wheel assembly.[1]

[edit] Tire geometry

When referring to the purely geometrical data, a shortened form of the full notation is used. To take a common example, 195/55R16 would mean that the width of the tire is 195 mm at the widest point, the height of the side-wall of the tire is 55% of the width

(107 mm in this example) and that the tire fits 16 inch diameter wheels. The code gives a direct calculation of diameter.

Less commonly used in the USA and Europe (but often in Japan for example) is a notation that indicates the full tire diameter instead of the side-wall height. To take the same example, a 16 inch wheel would have a diameter of 406 mm. Adding twice the tire height (2×107 mm) makes a total 620 mm tire diameter. Hence, a 195/55R16 tire might alternatively be labelled 195/620R16.

Whilst this is theoretically ambiguous, in practice these two notations may easily be distinguished because the height of the side-wall of an automotive tire is typically much less than the width. Hence when the height is expressed a percentage of the width, it is almost always less than 100% (and certainly less than 200%). Conversely, vehicle tire diameters are always larger than 200 mm. Therefore, if the second number is more than 200, then it is almost certain the Japanese notation is being used—if it is less than 200 then the U.S./European notation is being used.

[edit] Examples

The tires on a Mini Cooper might be labeled:

P195/55R16 85H

"P"—these tires are for a passenger vehicle. 195—the width of the tire is 195 mm at the widest

point. 55—indicates that the height of the side-wall of the

tire is 55% of the width—107 mm. R—this is a radial tire. 16—this tire fits 16 inch diameter wheels. 85—the load index, a maximum of 515 kg per

wheel in this case. H—the speed index, this means the maximum

permitted speed, here 210 km/h (130 mph).

The tires on a Hummer H1 might be labeled:

37X12.5R17LT

37-the tire is 37 inches in diameter. 12.5-the tire has a cross section of 12.5 inches R-this is a radial tire.. 17-this tire fits 17 inch diameter wheels. LT-this is a light truck tire.

for the BF Goodrich Radial T/A

Tire SizeServ.Desc.

Sidewall MSPNStock

#

Rim WidthRange

(inches)

*SectionWidth

OverallDiameter

Tread Depth

(in /32's)

Rev's permile

Max. load(lbs.@psi)

P175/70R13 82S RWL 46604285-308

4.5 - 6.0

7.0 on 5.0

22.7 12.0 916.0 1036@35

P195/60R13 83S RWL 81573265-305

5.5 - 7.0

7.9 on 6.0

22.2 11.0 937.0 1080@35

P205/60R13 86S RWL 55222265-309

5.5 - 7.5

8.2 on 6.0

22.7 11.0 916.0 1179@35

P215/50R13 84S RWL 91781275-307

6.0 - 7.5

8.9 on 7.0

21.5 11.0 967.0 1091@35

P185/70R14 87S RWL 50618285-410

4.5 - 6.0

7.4 on 5.5

24.3 12.0 856.0 1201@35

P195/70R14 90S RWL 48578285-413

5.0 - 6.5

7.9 on 6.0

24.8 12.0 839.0 1312@35

P205/70R14 93S RWL 66748285-416

5.0 - 7.0

8.2 on 6.0

25.4 12.0 819.0 1433@35

P215/70R14 96S RWL 50228285-420

5.5 - 7.0

8.7 on 6.5

25.9 12.0 803.0 1554@35

P225/70R14 98S RWL 79604285-459

6.0 - 7.5

9.0 on 6.5

26.5 12.0 785.0 1675@35

P195/60R14 85S RWL 84497265-402

5.5 - 7.0

7.9 on 6.0

23.2 11.0 897.0 1135@35

P215/60R14 91S RWL 98659265-418

6.0 - 7.5

8.7 on 6.5

24.2 11.0 860.0 1345@35

P225/60R14 94S RWL 53273265-434

6.0 - 8.0

9.0 on 6.5

24.7 11.0 842.0 1455@35

P235/60R14 96S RWL 79644265-477

6.5 - 8.5

9.5 on 7.0

25.1 11.0 829.0 1565@35

P245/60R14 98S RWL 54322265-486

7.0 - 8.5

9.8 on 7.0

25.6 11.0 813.0 1675@35

P245/50R14 93S RWL 48000275-536

7.0 - 8.5

10.0 on 7.5

23.7 11.0 878.0 1433@35

P265/50R14 98S RWL 91337275-559

7.5 - 9.5

10.9 on 8.5

24.5 11.0 849.0 1642@35

P155/80R15 83S RWL TBD TBD TBD TBD TBD TBD TBD TBD

P215/70R15 97S RWL 72634 285- 5.5 - 8.7 on 26.9 12.0 773.0 1620@35

639 7.0 6.5

P225/70R15 100S RWL 67244285-523

6.0 - 7.5

9.0 on 6.5

27.4 12.0 759.0 1753@35

P235/70R15 102S RWL 61977285-543

6.0 - 8.0

9.5 on 7.0

28.0 12.0 743.0 1896@35

P255/70R15 108S RWL 45982285-561

6.5 - 8.5

10.2 on 7.5

29.1 12.0 715.0 2183@35

P285/70R15 115S RWL 75701285-145

7.5 - 9.5

11.3 on 8.5

30.8 12.0 675.0 2679@35

P215/65R15 95S RWL 78624265-587

6.0 - 7.5

8.7 on 6.5

26.0 10.5 800.0 1510@35

P195/60R15 87S RWL 64472265-487

5.5 - 7.0

7.9 on 6.0

24.2 11.0 860.0 1190@35

P205/60R15 90S RWL 71072265-515

5.5 - 7.5

8.2 on 6.0

24.7 11.0 842.0 1301@35

P215/60R15 93S RWL 67797265-510

6.0 - 7.5

8.7 on 6.5

25.2 11.0 825.0 1411@35

P225/60R15 95S RWL 64148265-534

6.0 - 8.0

9.0 on 6.5

25.6 11.0 813.0 1521@35

P235/60R15 98S RWL 54712265-531

6.5 - 8.5

9.5 on 7.0

26.1 11.0 797.0 1642@35

P245/60R15 100S RWL 60013265-552

7.0 - 8.5

9.8 on 7.0

26.6 11.0 782.0 1753@35

P255/60R15 102S RWL 71728265-562

7.0 - 9.0

10.2 on 7.5

27.1 11.0 768.0 1885@35

P275/60R15 107S RWL 45185265-564

7.5 - 9.5

11.0 on 8.0

28.0 11.0 743.0 2149@35

P205/55R15 87S BSW 88561270-321

5.5 - 7.5

8.4 on 6.5

23.9 11.0 870.0 1201@35

P265/50R15 99S RWL 88000275-744

7.5 - 9.5

10.9 on 8.5

25.5 11.0 816.0 1720@35

P275/50R15 101S RWL 50381275-771

7.5 - 9.5

11.2 on 8.5

25.9 11.0 803.0 1830@35

P295/50R15 105S RWL 53415275-777

8.0 - 10.0

12.2 on 9.5

26.7 11.0 779.0 2061@35

P275/65R16 111S RWL 93892265-240

7.5 - 9.5

11.0 on 8.0

30.1 10.5 691.0 2403@35

P265/60R16 106T RWL TBD TBD TBD TBD TBD TBD TBD TBD

P285/60R16 111T RWL 93229 TBD TBD TBD TBD TBD TBD TBD

P205/55R16 89S BSW 70523270-365

5.5 - 7.5

8.4 on 6.5

24.9 11.0 835.0 1279@35

P195/50R16 83S BSW 60277270-555

5.5 - 7.0

7.9 on 6.0

23.7 11.0 878.0 1074@35

P205/50R16 86S BSW 89159270-222

5.5 - 7.5

8.4 on 6.5

24.1 11.0 863.0 1168@35

P225/50R16 91S BSW 67125270-250

6.0 - 8.0

9.2 on 7.0

24.9 11.0 835.0 1367@35

P245/50R16 96S BSW 53281270-803

6.5 - 8.5

10.0 on 7.5

25.7 11.0 809.0 1576@35

P265/50R16 101S BSW 66711270-229

7.5 - 9.5

10.9 on 8.5

26.5 11.0 785.0 1797@35

P295/50R16 107S BSW 51487270-239

8.0 - 10.0

12.2 on 9.5

27.6 11.0 754.0 2149@35

*The dimensions shown are average values for tires measured on the specified measurig width.**Section width varies approximately 0.2" for every 0.5" change in rim width.

* * * * *

Tire Weight database

(Originally provided by Jesse Lee)

Tire 205/60/13 185/60/14 195/50/15 205/40/16 205/45/16

Avon ZZ1         20

Avon CR338   17      

BFG Radial TA 20.18        

BFG Comp TA ZR     21.06    

BFG Euro TA     20.50 19.89  

BFG COMP TA R1 20.55 20.11      

BFG COMP TA HR4 Gen2

  18.76      

BFG Scorcher TA       20.42  

Bridgestone RE71     18    

Bridgestone RE730     ?    

Bridgestone RE920   15 18   20

Bridgestone RE930   17      

Bridgestone RE950   18.3      

Bridgestone RE92   17      

Bridgestone Eager   18      

Bridgestone Potenza S-03 PP

    20.7    

Bridgestone <others>          

Cooper Cobra Radial GT

18 17 19    

Cooper Cobra GTH   17      

Cooper Cobra XST   18      

Dunlop SP9000     18.1   17.8

Dunlop SP8000     20.3    

Dunlop D60 A2   17.4      

Dunlop W-10         19.6

Dunlop GT Qualifier 18 17 19 18  

Dunlop <others>          

Falken FK06U discont' ?      

Falken GRB     19.0    

Falken ZE 502 HR 17.0 ? 17.6 18.1  

Falken ZE 502 VR     18.1    

Firestone FTX   16      

Firestone GTA   17      

Firestone SH30   16      

Firestone SS2015 (175/70

/13) 16 17    

Firestone SS10   19 17    

Firestone (others)          

Goodyear GSD2         ?

Goodyear GSD3      18.7 (215/40-16)

 

Goodyear GT II   ? ?    

Goodyear HP U         ?

Goodyear HP   ?      

Hoosier Sport Car DOT Bias

14        

Hoosier Sport Car DOT Radial

16.5 14.5      

16 (195/55 /14)

16 (205/55 /14)

Kumho HP4   16.1      

Kumho Ecsta 711  avail. Q3'00

18.7   ?

Kumho Ecsta 712     18.2 19.821 (215/40-ZR16)

Kumho VictorRacer

V700 20.0       19.95

Michelin Pilot MXX3         24.29

Michelin Pilot XGT V     18.74    

Michelin XGT H4   18.09      

Nitto 450     17.6 18.3  

Nitto <others>          

Pirelli P0 A         18.7

Pirelli P7000     17.6   18.6

Pirelli P700 Z     18.3    

Toyo T1-S     17.418.1 (215/ 40/ 16)

18.1

Toyo Proxes Z1     20    

Toyo Proxes RA1 20        

Toyo Proxes FZ4     17 19 22

Toyo Proxes H4   17      

Toyo <others>          

Yokohama A032R   18.56      

Yokohama A008RS 19.97   19.58    

Yokohama A008RS II   16.78      

Yokohama AVS Intermediate

21.66

18

20.88 (205 /55/14)

19.54  dis- continud (20.85)

Yokohama A520       19.84 19.84

 

Also see the Wheel Weight Tables

Back to the FAQ9 January, 2003

 

Common Metals in Road Runoff

Lead: leaded gasoline, tire wear, lubricating oil and grease, bearing wear Zinc: tire wear, motor oil, grease, brake emissions, corrosion of galvanized

parts Iron: auto body rust, engine parts Copper: bearing wear, engine parts, brake emissions Cadmium: tire wear, fuel burning, batteries Chromium: air conditioning coolants, engine parts, brake emissions Nickel: diesel fuel and gasoline, lubricating oil, brake emissions Aluminum: auto body corrosion

Abstract: Tire-tread material has a zinc (Zn) content of about 1 wt %. The quantity of tread material lost to road surfaces by abrasion has not been well characterized. Two approaches were used to assess the magnitude of this nonpoint source of Zn in the U.S. for the period 1936-1999. In the first approach, tread-wear rates from the automotive engineering literature were used in conjunction with vehicle distance-driven data from the U.S. Department of Transportation to determine Zn releases. A second approach calculated this source term from the volume of tread lost during lifetime tire wear. These analyses showed that the quantity of Zn released by tire wear in the mid-1990s was of the same magnitude as that released from waste incineration. For 1999, the quantity of Zn released by tire wear in the U.S. is estimated to be 10 000-11 000 metric tons. A specific case study focused on Zn sources and sinks in an urban-suburban watershed (Lake Anne) in the Washington, DC, metropolitan area for a time period of the late 1990s. The atmospheric flux of total Zn (wet deposition) to the watershed was 2 microg/cm2/yr. The flux of Zn to the watershed estimated from tire wear was 42 microg/cm2/yr. The measured accumulation rate of total Zn in age-dated sediment cores from Lake Anne was 27 microg/cm2/yr. These data suggest that tire-wear Zn inputs to urban-suburban watersheds can be significantly greater than atmospheric inputs, although the watershed appears to retain appreciable quantities of vehicular Zn inputs.

Publisher: American Chemical Society

The atmospheric pressure on the surface of the Earth averages 101.325 kPa, with a scale height of about 8.5 km.[65] It is 78% nitrogen and 21% oxygen, with trace amounts of water vapor, carbon dioxide and other gaseous molecules

Introduction

Enrico Fermi (1901-1954) is a very famous

modern physicist. Under his guidance, human

have created the first atomic bomb and

successfully controlled thermonuclear reactions,

leading the world into an atomic era. His life

was very interesting, and the most well-known

events concerned his ability to make order of

magnitude estimation in physical problems. It is

said that when first atomic bomb in the world

experimentally exploded, Fermi shed paper

debris at a great distance from the centre of

explosion, and carefully observed the drifting

distance of the debris under air pressure; after a few minutes of mental

arithmetic, he estimated that the power of explosion was about several ten

thousand tons of TNT. The estimation corresponds to the correct order

produced by precise instrument available a few weeks later, and this surprised

his colleagues a lot.

Question

Fermi liked using simplified problems to stimulate his students to make order

of estimation in physical problems. One of the interesting problems is: "When

you take a single breath, how many molecules of gas you intake would have

come from the dying breath of Caesar?" For the sake of simplicity, we can

assume that the molecules which Caesar exhaled in his last breath have diffused

evenly to the whole atmosphere, and these molecules were not absorbed by the

ocean or plants for thousands of years. Although these are not valid

assumptions, they can help us forget about the complexity of the real world,

and to make elementary estimations in the simplest way. For the convenience

of your calculation, we have given hints and information as follows:

Enrico Fermi

Hints

We shall at first estimate the ratio of the gas volume exhaled in a single breath

to the volume of the whole atmosphere (for simplicity, you may assume that all

gases are evenly distributed in a layer which has a thickness of 50 km on the

surface of the earth). Then you can estimate, in your single breath, how much

volume of gas would have come from Caesar's last exhalation, and finally, with

data on the density of the atmosphere and the average mass of an air molecule,

you can estimate the no of molecules that the volume contains.

Assume that Radius of the earth R = 6,400 km Volume of gas in a single breath

~ 1 litre Thickness of the atmosphere ~ 50 km Mass of a proton mass of a

neutron Density of the atmosphere on the surface of the earth

=

Reference: The Fermi Solution, Hans Christian von Baeyer

Answer

First we shall estimate the volume of the earth's atmosphere V. Since the

thickness of the atmosphere is much less than the radius of the earth, we have

The air intake in a single breath is about 1 litre, i.e., . Assuming that

the gas exhaled from Caesar's last breath is evenly distributed in the

atmosphere, we can deduce that, in a single breath, the volume of gas that one

intake from Caesar's last breath is

Nitrogen is the major component of the atmosphere of the earth, and oxygen

the second. Since the molecular masses of nitrogen and oxygen do not differ

much, we will simply use nitrogen in our estimation. A nitrogen molecule has

two atoms, each with 7 protons and 7 neutrons. Neglecting the mass of an

electron, a nitrogen molecule would have a mass of

Hence in a single breath, the number of the molecules that comes Caesar's last exhalation

would roughly be

i.e., when we take a single breath, we would have intake a single molecule

which comes from Caesar's last exhalation.

Note

Some students may notice that I have only taken one significant figure throughout my calculations. Frankly speaking, I have not used a calculator in any of the calculations above. From your letters, I notice that some students have tried to perform very accurate calculations, and some of them have even made an effort to analyze the composition of air. In fact, these are not necessary, because under the many assumptions that we have used to simplify our problem (e.g. we have assumed that the density of air is uniform, which is obviously not true), it is virtually impossible to obtain an accurate answer. As a matter of fact, the spirit of the "Fermi problem" lies on training us to deal with a problem in which detailed information and calculation techniques are not available, and yet we can still make a very rough, but barely reliable estimation to get the right order of magnitude.